Major chordate groups

Chordates belong to the animal phylum Chordata and include the vertebrates, together with
several closely related invertebrates, the urochordates and cephalochordates. Until recently, an
invertebrate group, the Hemichordata was placed under the phylum Chordata but is now
considered as a separate phylum. However, hemichordata will be considered along with the rest
invertebrate groups.
The four chordate subphyla i.e. Urochordata, Cephalochordata, Hemichordata (Protochordates)
and Vertebrata are so grouped on account of having certain primary features namely:
i) a notochord (a dorsal fairly rigid rod of vacuolated cells) or a vertebral column
ii) a hollow dorsal nerve cord or spinal cord
iii) pharyngeal slits
iv) a post-anal tail.

The three invertebrate subphyla (Hemichordata, Urochordata, Cephalochordata) mentioned

above are collectively called protochordates because they are considered to be the ancestral stock
of the chordates. In other words, they represent the primitive form of chordates.

Characteristics of Phylum Chordata

 Bilateral symmetry; segmented body; three germ layers; well developed coelom
 Notochord (a skeletal rod) present at some stage in the life cycle
 Single, dorsal, tubular nerve cord; anterior end of cord usually enlarged to form brain
 Pharyngeal pouches present at some stage in the life cycle; in aquatic chordates these
develop into pharyngeal slits
 Endostyle in floor of pharynx or a thyroid gland derived from the endostyle
 Postanal tail projecting beyond the anus at some stage but may or may not persist
 Complete digestive system
 Segmentation, if present, restricted to outer body wall, head, and tail and not extending
into coelom
Body Outline of Chordates

The generalised body structure of a typical chordate

The phylum Chordata consists of three subphyla: Urochordata, which is represented by tunicates;
Cephalochordata, represented by lancelets and Vertebrata represented by the vertebrates. The
fourth subphylum Hemichordata presented by the acorn worms.

Hemichordata

Hemichordates are described as half chordates because of the primitive nature of their notochord.
That explains why they are excluded from the Phylum Chordata by most authors/taxonomists.
However, following the traditional classification, we have included the group amongst the
protochordates (i.e. the Hemichordata, Urochordata and Cephalochordata).

Characteristics

 Body is divided into three sections - proboscis, collar and trunk

 Body is bilaterally symmetrical

 Primitive notochord is restricted to the proboscis only and thus called stomochord
 Body has more than two cell layers, tissues and organs
 A true coelom (body cavity)
 A straight or U-shaped gut, with an anus
 Nervous system normally diffuse, but variable
 A partially open circulatory system
 Glomerulus as excretory organ
 Reproduction normally sexual
 Feed on fine particles in the water
 Live in marine environments.

As we have described above, the hemichordates are distinguished by a tripartite (three parts)
division of the body. At the anterior end of the body is a proboscis (pre-oral lobe); behind this
are a collar, and lastly a trunk. The hemichordates share some (but not all) of the typical chordate
characteristics. In some DNA-based studies of evolution suggest that hemichordates are actually
closer to echinoderms than to true chordates. This is supported by the fact that the larvae of at
least some hemichordates look very much like those of some echinoderms.

Assignment: Briefly describe Acorn worms and Pterobranch

Urochordates

They represent the most primitive of the true chordate.

Characteristics

 notochord present only at the developmental tadpole stage; absent in the adult stage thus, adult
has no endoskeleton
 hollow nerve cord
 post anal tail
 body wholly covered by a structure called 'tunic' made of secreted protein and cellulose-like
material

 body has more than two cell layers and includes tissues and organs - triploblastic
 U-shaped gut
 body without coelomic body cavity
 hermaphroditic with external fertilisation
 nervous system composed of an anterior ganglion from which individual nerves issue arise
 no excretory organs
 a distinct larval stage that undergoes metamorphosis
 gill slits are used to trap food particles during filter feeding
 ventral heart present with incomplete closed circulatory system
 haemocyanin as blood pigment (no hemoglobin)
 habitat is marine environment.

Urochordates are a medium sized group of marine animals commonly referred to as Sea squirts,
Tunicates, Salps or Larvaceans. They are all filter feeders using a basically similar mechanism of
pumping water through a perforated pharynx, which collects small particles in a layer of mucus.
All the urochordates have an external covering or 'house' called the tunic, which is made of
secreted proteins and a polysaccharide much like cellulose. In some cases, this matrix contains
living cells that have migrated from the main body of the animal, and even sometimes blood
vessels. The animal lives within its ‘house’ permanently in most cases. The 'houses' of the
larvaceans are less substantial as new ones are secreted every four hours or so. Although the
urochordates are close relatives of the chordates and also of vertebrates such as mammals, they
seem to be far less like vertebrates than many of the other invertebrate phyla; i.e. they have no
limbs, no brain and except in the larvaceans, the tail is only evident during larval development.
The subphylum is divided into three classes namely: Ascidiacea, Thaliacea and Larvacea.

The class Ascidiacea has the following characteristics:

 notochord and post-anal tail found in the larval stage only.
 tadpole-like larvae metamorphose into adults

 sessile (non-moving or staying in one place) adults

 marine habitat - most species are common coastal animals occurring in rock pools and out into
deeper water to about 400-5,000 m in depth
 solitary or colonial - the colonial species may share a common exhalent siphon.
 translucent or whitish body colour but some species are much more colourful and can be red,
brown, yellow and even blue
 tunic composed mostly of an acellular (not made of cells) matrix of tunicin, a polysaccharide
similar to cellulose (there are some living cells of various sorts within this matrix but they are
well spaced out)
 pharynx has numerous small pores or slits in its walls for the passage of water
 two openings, an inhalant siphon (where water comes in) and an exhalent siphon (where water
goes out)
 hermaphroditic - male and female reproductive organs in each individual
 filter feeders.
 Structure of a common tunicate

White Colony of Sea Squirts

The name tunicate arises from the existence of the tunic (external covering or 'house') while Sea
squirt arise from the fact that when squeezed, water shoots out of the exhalent siphon. The larvae
swim towards light at the surface of the sea at first then after a short while they reverse direction
and swim down towards the sea floor, often in less than one day. Tunicate larvae do not feed and
are essentially a dispersal form. They soon find a suitable spot on the sea floor and settle in a
head down, tail up position. They attach themselves to the sea floor (substrate) using special
adhesive glands in the front of their head and then undergo an amazing metamorphosis during
which the post-anal tail and the notochord are lost. The remainder of the body twists through 180
degrees to form a small tunicate. Tunicates feed by drawing water in through the inhalant siphon.
This water passes through the pharynx where small particles are trapped before the water leaves
the body through the exhalent siphon. The water current is maintained by beating cilia, though
they can force water out of the atrial cavity by muscular contraction of the tunic, if frightened.
The small particles, plankton and so on, are trapped on a continually moving layer of mucous.
This mucous is secreted by special cells and is moved across the surface of the pharynx by the
beating of many small cilia, eventually it is passed in the digestive tract where both the particles
and it caught up in it are digested.

Assignment: Briefly write on Apendicularia

Cephalochordata

The cephalochordates are the most advanced protochordates, clearly showing the four primary
features of chordates (notochord, dorsal nerve cord, pharyngeal gill slits and post anal tail)
throughout their life. They are the closest to the vertebrates.

Characteristics

 Notochord: well-developed and persists throughout life of the animal. It runs the length of the
animal from the tail to the tip of the nose on the head - a feature that gives subphylum its name
(cephalo- meaning head).
 numerous gill slits over 100 used to trap food particles during filter feeding
 dorsal nerve cord
 post-anal tail
 marine and fish-like in appearance (both ends pointed)

 no normal vertebrate endoskeleton

 external fertilisation
 some metamerism (body segmentation) in the musculature
 no heart
 use haemocyanin pigment (no haemoglobin)
 closed blood circulatory system.

The subphylum Cephalochordata is usually represented by one organism Branchiostoma

commonly called Amphioxus (which means "sharp at both ends") or lancelets. Cephalochordates
are like vertebrates in having the derived feature of an elongated body as adults, but are still
(primitively) filter feeders; that is, they feed while motionless, moving food-laden water by
means of cilia on their gill bars.
Amphioxus is 51-76 mm (2-3 in) in length and whitish to creamy yellow, sometimes with a tint
of pink. It lives on seashores throughout the temperate zone. Amphioxus shows some
cephalisation, in that the primary feeding structures are concentrated at the anterior end, and it
has a light sensitive pigment spot on the anterior end that may be used for orienting it toward
light.
Amphioxus is fish-like in appearance but without eyes. It has a laterally compressed dorsal fin.
There are about 25 species of cephalochordates inhabiting shallow tropical and temperate
oceans; they spend much of their time buried in the sand of ocean beds.
Lancelets have a notochord - a flexible rod of cells supporting the body.
The rest of the skeleton is made up of small, flexible rods between the gill slits and supporting
the mouth bristles. A dorsal nerve cord runs along the top of the notochord. They have blood
vessels but no heart.
The pharynx is perforated by over 100 pharyngeal slits or "gill slits", which are used to strain
food particles out of the water. The musculature of the body is divided up into V-shaped blocks
or myotomes, and there is a post-anal tail. All of these features are shared with vertebrates. On
the other hand, cephalochordates lack features found in most or all true vertebrates; the brain is
very small and poorly developed and the sense organs are also poorly developed, and there are
no true vertebrae (bone in spinal column).

Cephalochordate (Amphioxus)
 Cephalochordate (Amphioxus)

The subphylum Cephalochordata is comprised of small marine organisms that exhibit the basic
chordate features throughout their life. They have a well-defined notochord, numerous gill slits,
dorsal nerve cord and post-anal tail. They exhibit well-defined body segmentation and
cephalisation.

The Main features of vertebrates as main chordate groups

In Major chordate groups, we discussed the ancestral chordates called the protochordates,
comprising the Hemichordata, Urochodata and the Cephalochodata. You will recall that
the major structural support of this group of animals (protochordates) is the notochord,
which manifests at some stage or throughout their life. In this topic, we shall be looking
at the characteristic features of the advanced chordates – the vertebrates. You will also
recall that we said that a major departure of the vertebrates from their ancestral stock was
the replacement of the notochord with a vertebral column (the backbone) from which the
name of the subphylum is derived. The vertebral column may be cartilaginous in nature
as is the case in some fishes or bony as in most vertebrates. Unlike the protochordates,
vertebrates are the ones we oftentimes come across more or less on a daily basis. Indeed,
in most homes, some of these animals are part of the family. Such animals include dogs,
cats, horses, chickens, pigeons, etc. It must be emphasised that we humans are chordates.

The chordate subphylum, Vertebrata, is characterised by the following basic

features:
Notochord is not present in adult; it is replaced by spine of cartilaginous or bony column
- the vertebrae/backbone.
A complex brain encased by a cranium, which protects and supports it.
Well-developed head (cephalisation) with advanced nervous and sensory structures.
Most have two pairs of appendages: one pair of pectoral and one pair of pelvic
appendages.
Other features
Bony and/or cartilage endoskeleton for structural support and or locomotion.
True body cavity – the coelom.
Males and females are separate and distinct.
Gill slits are few in number, when present.
Variety of feeding strategies: herbivores, carnivores, omnivores, filter feeders, parasites.
Well-developed ventral heart with 2-4 chambers.
Closed circulatory system with haemoglobin as the respiratory pigment in the blood.
Variety of habitats including freshwater, salt water, terrestrial.
Specialised epidermal structures in the form of scales, feathers, hair, fur, spines.
True kidneys.
Efficient respiratory system of gills or lungs.
Body is bilaterally symmetrical and of three parts - head (with internal skeleton the
cranium), trunk and post-anal tail.

Vertebrate Embryos

Vertebrates that evolved from fish pass through similar embryonic stages. A flexible
notochord develops in the back and blocks of tissue called somites form along each side
of it. These somites will become major structures, such as muscle, vertebrae, connective
tissue, and, later, the larger glands of the body. Just above the notochord lies a hollow
nerve cord.
Vertebrates are chordates with a well defined backbone (cartilaginous or bony), complex
brain encased by a cranium, well developed head (cephalisation) with advanced nervous
and sensory structures, and two pairs of appendages (1 pair of pectoral and 1 pair of
pelvic appendages).

Characteristics of the Superclass Agnatha

Members of the superclass have the following features:
lack jaws hence the name agnatha (without jaw)
vertebral spine is cartilaginous in nature
head with a cranium that encases a brain
mouth is generally round/ circular
no scales or exoskeleton.

Characteristics of the Class Ostracodermi (Extinct)

This class is extinct but had the following characteristics:
small fish-like animals (only few centimeters long)
bottom dwellers, poor swimmers
rudimentary fins and bony armor
no lower jaw
no teeth
filter feeders or deposit feeders
marine.

The Three Types of Ostracoderms

Characteristics of the Class Cyclostomata (lampreys and hagfishes)

The class cyclostomata derived its name by having a round or circular mouth. The class is
characterised by the following features:
eel-like in shape
lack jaws but have rows of horny teeth that move in circular motion and give the mouth
a circular shape - hence the name cyclostomata. In the absence of a jaw, the mouth cannot
close and is always open such that water constantly cycles through it.
prey/parasitise on fishes
lack exoskeleton/scales
notochord persists in adults
marine habitat with size of 10-90cm in length.

The cyclostomes are very unique among vertebrates because of their semi-parasitic
nature. The lampreys, with the exception of some small freshwater forms, attach
themselves to other fishes using their suctorial mouth and then rasp off the flesh by
means of the horny teeth carried by the highly-developed tongue. The hagfishes are
capable of boring their way right into the body of their prey, devouring all the soft parts
and leaving the skin behind as an ordinary empty shell, held by the bones. In large
numbers, lampreys can cause great damage to fisheries especially fishes caught by hooks
or nets as they eat up their flesh leaving them as empty shells as described above.

The class Cyclostomata consists of two orders: Petromyzontia (or Hyperoartii) e.g.
lampreys and the Myxinoidea (or Hyperotreti) e.g. hagfishes.

A Cyclostome - Sea Lamprey

 A Cyclostome - Hagfish Tie Self in Knot

Superclass Gnathostomata
The superclass Gnathostomata, which contains vertebrates with jaws. Human, fish, dogs,
cows, goats, cats and other vertebrates we see most often, have jaws; human and these
animals are therefore gnathostomes. We shall therefore be examining a complex yet
exciting group of animals that we consume as food, keep as pets, exploit for services or
view as wild animals, etc. Our starting point is the fishes, which we all know dwell in
water.

Gnathostomes are characterised by:

a vertically biting device called jaws (primitively made up of two endoskeletal
elements, the palatoquadrate and Meckelian cartilage, and a number of dermal elements
called teeth, sometimes attached to large dermal bones)
paired appendages (paired pectoral and pelvic fins) supported by an internal skeleton,
which supports more efficient locomotion
interventrals and basiventrals in the backbone. These are the elements of the backbone,
which lie under the notochord, and match the basidorsals and interdorsals respectively.
gill arches, which support/hold the gills, lie internally to the gills and branchial blood
vessels, unlike the gill arches of all jawless craniates, which are external to the gills and
blood vessels
a horizontal semicircular canal in the inner ear
teeth - modified dermal scales
paired nasal sacs.
The Gnathostomata or gnathostomes differ from all other craniates or vertebrates in
having a vertically biting device, the jaws, which consist of an endoskeletal mandibular
arch and a variety of exoskeletal grasping, crushing, or shearing organs, i.e. the teeth, and
jaw bones. The gnathostomes include sharks, rays, chimaeras, ray-finned fishes,
lobefinned fishes and land vertebrates including humans.

Characteristics of the Class Placodermi (Extinct)

The class Placodermi is regarded as the first set of fish with jaws but are now extinct.
The members of this class had the following characteristics:
well-developed fins and armour plating
dermal armour consisting of head armour and thoracic armour. In the thoracic armour,
the foremost dermal plates form a complete "ring" around the body and always include at
least one median dorsal plate.
inhabit both freshwater and marine environment
bottom dwellers.
large size (up to 10 m)
lower jaw and teeth present

A Typical Outlook of Placorderms

Characteristics of the Class Chondrichthyes (Cartilaginous fish)

The cartilaginous fishes have the following characteristics:
internal skeleton is composed of cartilage (rather than bone)
body scales are placoid (tooth-like) with a bony base
jaws suspended by two gill arches
swim bladder or lung absent; have oil-filled liver to provide buoyancy
claspers (modified pelvic fins) present in males for internal fertilisation
notochord present in the young and gradually replaced by a backbone of cartilage in
adult
ventral mouth
gills not covered by operculum
fleshy pectoral and pelvic fins.
The class Chondrichthyes is divided into two extant subclasses:
Subclass: Elasmobranchii (sharks, rays and skates)
Subclass: Holocephali (chimaera, sometimes called ghost sharks).

Members of the class Chondrichthyes have a backbone that is made of cartilage. They are
not the fishes you come across often because of their marine.

Characteristics of the Class Osteichthyes (Bony fish)

Members of this class have the following characteristics:
bony endoskeleton
body covered by cycloid scales (thin and round bony scales)
paired pectoral and pelvic fins supported by bony rays
bilaterally symmetrical tail fin
visceral cleft as separate gill openings covered by a bony flap –the operculum.
The Osteichthyes are characterised by endochondral ("spongy") bone in the endoskeleton,
dermal fin rays made up by lepidotrichiae (modified, tile-shaped scales), and three pairs
of tooth-bearing dermal bones lining the jaws (dentary, premaxillary and maxillary). The
Osteichthyes include two major subclasses, the Actinopterygii and the Sarcopterygii.

Characteristics of the Class Amphibia

The amphibians are characterised by the following features:
moist, glandular skin that lacks the keratinised scales of reptiles
complex life cycles (eggs, tadpole/juveniles, adults through metamorhosis)

non-amniotic eggs (they lack the amniotic membrane that surrounds the embryo)
eggs lack a shell instead surrounded by several gelatinous layers
gills at the larval stage and lungs at the adult stage. In many amphibians, the skin is
also important in gas exchange
two pairs of pentadactyl (five digits) limbs
cold-blooded animals (they do not have a constant body temperature but instead take
on the temperature of their environment)
three-chambered heart
no external ear.
The moist, scale-less skin of amphibians absorbs water and oxygen from the surrounding
atmosphere, but that also makes them vulnerable to dehydration (loss of bodily fluids).
Without moist conditions, their skin dries out and they die. That explains why
amphibians are most often found near ponds, marshlands, swamps, and other areas where
moisture is available. Some amphibians become inactive when conditions are
unfavourable for survival. This period of inactivity is called estivation when it occurs
during hot, dry weather and hibernation when it occurs in response to cold temperatures.
Activity resumes when favourable conditions return.
The thin skin of amphibians contains many glands; among them is the poison gland that
protects certain species against predators. The poison from the glands of the brightly
coloured poison-dart frog is particularly toxic and is used by South American Indians to
coat the tips of their arrows. Some amphibians protect themselves from enemies by
changing colour to blend in with their surroundings i.e. they camouflage.
The life cycle of most amphibians begins in water when the female lays eggs that are
fertilised outside of her body. The eggs then hatch into larvae (known as tadpoles), that
breathe through external gills. The larvae grow flat tails and feed on vegetation. During a
process called metamorphosis, physical changes occur and external gills give way to
lungs. The tadpoles also change from plant-eating (herbivorous) to meat eaters
(carnivorous) animals. Amphibians usually reach full adulthood at three to four years.
The class Amphibia is comprised of three orders which include the Gymnophiona
(caecilians), Urodela (urodeles - newts and salamanders) and Anura (anurans - frogs and
toads).

Class Reptilia
Some reptiles live in water but return to land to lay eggs, unlike the amphibians whose
eggs are laid in water; reptile eggs have a shell/thick membranous covering that also
protects against desiccation. Furthermore, unlike the situation in amphibians, the embryo
in the eggs of reptiles is protected by a thin membrane called the amniotic membrane. So
we can rightly say that the amniotic membrane finds its genesis in reptiles. The ability of
reptiles to colonise land is also supported by efficient lungs at the adult stage, which
enhance respiration as against the use of gills and moist skin as obtains in many
amphibians. Reptiles are animals we see quite often – the most common being the lizards.
Others include wall geckos, crocodiles, snakes and tortoises.

Characteristics of the Class Reptilia (Lizards, crocodiles, snakes, tortoises,

turtles)
Reptiles are characterised by the following features:
dry skin with keratinised epidermal horny scales
bony endoskeleton
two pairs of pentadactyl (five digits) limbs with true claws (if limbs are present)
no external ear
fertilisation is internal and fertilised eggs laid (oviparous) on land or eggs retained
internally until hatching (ovoviviparous)
amniotic egg with leathery shell

Typical Amniotic Egg

cold blooded (poikilothermic/exothermic)

gut and the ducts of the urinary and reproductive system open into a posterior chamber
called the cloaca.
There are about 6,550 living species of reptiles worldwide; they live in a wide range of
habitats, including forests, swamps, grasslands, deserts, oceans, and mountains. The
name "reptile" is generally applied to any of a group of ectothermic (cold-blooded i.e.
need an "outside" source of heat to generate adequate body heat) vertebrates in the Class
Reptilia.
Reptiles must regulate their body temperature by behaviour, either by basking in the sun
to keep warm or by hiding under cover to keep cool. Some reptiles (such as lizards) look
superficially similar to the tailed amphibians (newts and salamanders). However, reptiles
have dry and not moist skin. They are covered in scales or a shell. If they have legs,
they have true claws on their toes. Reptiles can be far from water sources because their
skin retains water better than that of amphibians.
Reptiles lay amniotic eggs that have a leathery shell that prevents rapid water loss.

The Class Reptilia is composed of four orders namely Order Crocodilia (crocodiles and
alligators), Order Testudinata (turtles), Order Squamata (lizards and snakes) and Order
Rhynchocephalia (Tuataras).

Characteristics of Order Crocodilia (Crocodiles and alligators)

The crocodilians have the following features:
have a long snout
four well-developed limbs
a muscular tail used to propel them through the water
lay eggs in large mounded nests or in cavities dug in the soil
carnivorous on fish, amphibians, reptiles, birds, and mammals.

A crocodile has a very long, narrow, V-shaped snout, while the alligator's snout is wider
and U-shaped. A crocodile's upper and lower jaws are nearly the same width, so the teeth
are exposed all along the jaw line in an interlocking pattern, such that even when the
mouth is closed the bottom teeth are visible. An alligator’s teeth don’t show when its
mouth is closed.

Characteristics of Order Testudinata (Turtles and tortoises)

The order is characterised by the following features:
shell or carapace formed from the fusion of vertebrae and ribs with dermal bones
no teeth but have a sharp-edged beak, called a tomium used as cutting edges to bite off
chunks of food
oviparous and fertilisation internal and accomplished by a penis which is an outgrowth
of the cloacal wall. Eggs are buried in a nest and left to incubate and hatch.

no temporal opening in the skull behind the eye, a condition known as anapsis. This
feature is unique among living reptiles.
The shell of testudinata is covered with scutes. No other vertebrate has the hard shell that
surrounds and protects the organs of turtles/tortoise. The shell of turtle/tortoise consists of
two basic parts, the top shell which is referred to as a carapace, and a bottom shell that is
known as a plastron. The two parts of the shell are connected on each side by a portion of
the shell known as the bridge. The ribs and vertebrae of turtle/tortoise, with the exception
of the neck and tail, are fused to form the carapace. Because of the fusion, you cannot see
a clearly defined vertebrae/backbone as is the case in other vertebrates we have studied so
far. The shell is not an exoskeleton as some people mistakenly assert, but a modified
ribcage and part of the vertebral column. It cannot be "taken off". Because of the shell,
the pectoral and pelvic girdles are uniquely located within the ribcage. The limb bones
are also modified to accommodate the shell. Turtles/tortoises are long-lived animals.
Some live from 20 to over 100 years, depending on species. Some species only eat
animal matter while others eat both plants and animals. The Latin word-root "test" is
synonymous for shell, and the order name "Testudines" is Latin for turtle. Tortoises are
considered as turtles that inhabit land and have un-webbed feet unlike the water turtle that
live most of the lives in water and have webbed feet. Both lay eggs on land. The two are
thus distinguished by their habitat i.e. land turtle or water turtle. However, the Americans
make no such distinctions as the word turtle refers to both, whether on land or water.

Characteristics of the Order Squamata (Lizards and

snakes)
The order is characterised by the following features:
transverse vent or cloacal opening
skull that is more moveable (or kinetic) than other reptile orders
paired copulatory organs called hemipenes
body covered in scales
periodically shed their skin (a process known as ecdysis/moulting)
carnivorous or omnivorous
variety of habitats (aquatic, terrestrial, or arboreal)
lay eggs; others bear live young (ovoviviparous).
The Order Squamata (meaning scaled reptiles) is the largest order of reptiles with over
6,000 living species. It is the most diverse of the reptile orders, containing 96% of the
reptile species. In other words, these are the reptiles you come across often. They are
represented by the Lacertilia (lizards) and the Serpentes (snakes). Members of this huge
order are found worldwide, except in Antarctica and on a few very remote islands. Many
squamates have loss or reduction of limbs and the ability to lose the tail (caudal
autotomy), especially when attacked by predators, as is the case with wall geckos. Snakes
(Serpentes) lack limbs; however, some species have vestigial (degenerate or functionless)
limbs in the form of small spurs (e.g. the rubber boa). All snakes lack eyelids and
external ear opening (some burrowing lizards lack ear openings as well). Snakes have an
elongate body (some lizard species are limbless and have long slender bodies). On the
other hand, lizards (Lacertilia) are characterised by four limbs (some lizards species that
lack limbs), visible ear openings, and movable eyelids. These three characters alone
readily distinguish lizards from snakes.

Characteristics of the Order Rhynchocephalia (Tuatara)

Members of this order are characterised by the following features:
a scaly loose skin which may be soft to touch
a spiny back

a third primitive, light-sensitive eye above the brain

live in burrows and are nocturnal, hunting at night just outside their burrow entrance
they feed on worms, lizards, millipedes and small seabirds.

Tuatara means "spiny back". Tuataras are reptiles but they are very different from lizards,
snakes, and crocodiles. Tuataras have a primitive body structure that supports the theory
that they are one of the oldest and most un-evolved species, having hardly changed in the
past 220 million years. The order has only two living species in New Zealand. They are
solitary, nocturnal hunters of insects and small vertebrates. Tuataras grow to 60
centimeters (24 inches) in length. They may live more than 50 years; their eggs must
incubate for 15 months before hatching—longer than for any other reptile

Class Aves

Birds represent a major departure from the reptiles as the body scales have been replaced
by feathers; this is a major characteristic feature of the class. Indeed, any animal you see
with feathers is undoubtedly a bird. They include the doves and pigeons on your roof and
on trees around your homes, to the chickens and turkeys you eat as delicacies, to the big
ostriches that cannot fly. Birds are better equipped to live on land than the reptiles.
Unlike the reptiles, they are warm blooded animals, have more efficient lungs with
pouches for gaseous exchange coupled with a heart of four chambers. Most birds fly but a
few such as penguins and ostriches have lost their ability to fly (though their ancestors
did fly). Birds also have a large-yolked egg encased in a hard calcareous shell that can
withstand desiccation.
Characteristics of the Class Aves (Birds)
body covered with feathers composed mainly of keratin (they are the only animals that
have feathers)

strong bony endoskeleton

bones with large air spaces
forelimbs modified as wings for flight (some have lost ability to fly)
bipedal- two legs for locomotion (lower part of legs has scales)
toothless horny beak; use gizzard to grind food
warm-blooded animals (body temperature is internally regulated; endothermic/
homeothermic)
efficient lungs with pouches for gas exchange
heart of four chambers
internal fertilisation
hard-calcareous shelled eggs with large yolk.
It is interesting to note that birds are the only vertebrates with feathers. So any animal
you see with feathers is undoubtedly a bird. Birds are said to have come from a common
ancestor (monophyletic lineage) and are thus related through that common origin.
Modern birds have traits related to high metabolism, ability to fly, a beak with no teeth,
laying of hard-shelled amniotic eggs, a four-chambered heart, a lightweight but
strong skeleton and forelimbs modified as wings. Birds also have unique digestive and
respiratory systems. Some birds, especially parrots, are among the most intelligent animal
species; a number of bird species have manufacturing skills and ability to tools, and many
social species exhibit cultural transmission of knowledge across generations. Adaptation
to flight in birds is facilitated by light body weight (a consequence of the absence of teeth
and ultra-light bones and air sacs), high body temperature, improved blood circulation,
high metabolism, and acute vision (that enables avoidance of danger such as tree
branches at high speed).

The class Aves is comprised of two subclasses namely Archaeonithes (extinct ancient
birds) and Neornithes (recent birds). So it is right to say that all living birds belong to the
subclass Neornithes, which is further divided into three Superorders namely:
Odontognathae (extinct), Paleognathae and Neognathae

Class Mammalia

Mammals, as we shall be considering shortly, are so termed on account of having

mammary glands (modified sweat glands) in females that produce milk for the newborn.
It is from these glands that the whole group takes its name 'Mammals'.
The class Mammalia has the following characteristics:
Main mammalian features:
Mammary glands in females for milk production for the newborn. This milk is produced by
modified sweat glands called 'mammary' glands. It is from these glands that the whole group
takes its name, 'Mammals'.
Hairy skin with keratin (for insulation) with sebaceous and sweat glands. All mammals have
some hair at least at the beginning of their lives - baby whales and dolphins are born with a
moustache.
A single jaw bone on either side. In all other vertebrates, there are more than one bones on each
side of the jaw.
Four-chambered heart with the main artery leaving the heart curves to the left becoming the
aortic arch. (In birds, it curves to the right and in all other vertebrates there are more than one
main artery leaving the heart).
Muscular diaphragm separates the thoracic cavity from the abdominal cavity.
Other mammalian features include:
bony endoskeleton
two pairs of pentadactyl limbs
outer ear lobe (pinna) plus middle and inner ears - three middle ear bones (the stapes or
stirrup, incus or anvil and the malleus or hammer)
warm-blooded (endothermic/homoeothermic - heat energy generated from within to
maintain a constant high body temperature)
seven cervical vertebrae (neck bones) are present in most mammals
viviparous; a few are oviparous
teeth (where present) are imbedded in jaw and in a variety of forms for different
functions (heterodontic)
internal fertilisation
well developed brain encased in a skull.
Mammals are divided into three main categories depending on how they are born. These
categories are monotremes, marsupials, and placentals.
Except for the five species of monotremes (which lay eggs), all mammal species give
birth to live young. Most mammals also possess specialized teeth, and the largest group
of mammals i.e. the placentals, use a placenta during gestation. The mammalian brain
regulates endothermic and circulatory systems, including a four-chambered heart.
Mammals have a buccal cavity (the mouth cavity) with a false palate as roof, which
means that the nostrils do not lead directly into the mouth.
Effectively, this means that if your mouth is full of food you can still breathe, but a
reptile has to breathe around food. The body temperature of mammals is maintained at a
constant level, meaning that mammals are endothermic i.e. they generate heat within their
bodies metabolically and also have special cooling mechanisms. This does not however
imply that all mammals maintain the same body temperature. There are approximately
5,400 species of mammals, distributed in about 1,200 genera, 153 families, and 29 orders
(though this varies by classification scheme). Mammals range in size from the 30-40-mm
(1.2-1.6 in) bumblebee bat to the 33 m (110 ft) blue whale.

Mammals are divided into two subclasses:

Subclass Prototheria: (monotremes: platypuses and echidnas) Members of this
subclass are extinct except the order Monotremata, which is represented by the duck-
billed platypus (flat footed) and the echidnas - spiny anteaters. This group of mammals
can be described as having transited from the reptiles as they exhibit some reptilian
features
as we shall highlight below.
The monotremes (as members are generally referred to) have the following
characteristics:
shelled eggs like reptiles
single opening – the cloaca (like reptiles) for both urine/faeces discharge and copulation
testes are not in scrotum but retained in body cavity
penis lies within the cloaca and although homeothermic, their body temperature is low,
from 30-33°C, so also is the basal metabolic rate
mammary glands (though not truly mammary glands), that produce a fatty sweat (milk)
from glands in the skin. The milk collects and drips down tufts of hair into the offspring's
mouth.

Subclass Theria (Live-bearing mammals)

Members of this subclass, unlike the monotremes described above, do not lay eggs but
give birth to live babies. This means that there is a period of development for the embryo
in the mother before birth. So, we can look at the therians as more advanced mammals
than the prototherians. The therians are further divided into two infraclasses namely –
Metatheria and Eutheria.