PROTOCHORDATES
Ernst Haeckel proposed the establishment of Phylum
Chordata
- Sp Urochordata, Sp Cephalochordata, Sp Vertebrata
Deuterostomata
Echinodermata (phylum)
Hemichordata (phylum)
Chordata (phylum)
Urochordata (subphylum)
Ascidiacea. Sea squirts (class)
Larvacea. Larvaceans (class)
Thaliacea. Thaliaceans (class)
Cephalochordata. Lancelet or Amphioxus (subphylum)
Craniata (subphylum = subphylum vertebrata of Haeckel)
Myxini. Hagfish
Vertebrata. Craniates w/ vertebrates
BASAL DEUTEROSTOMES (ECHINODERMS & HEMICHORDATES)
1. ECHINODERMS starfish, sea squirts, sea cucumber, sea daisies &
crinoid
- unique calcium carbonate skeleton
- secondary radial symmetry
2. HEMICHORDATES William Bateson added Enteropneusta to
phylum Chordata which he named Hemichordata
Dorsal nerve strand and the tendency to have a lumen in the
collar region believed by Bateson to be an expression of
the
same phenomenon that induces the formation of a
dorsal neural groove and tube in chordates
Have slits that open to the exterior basic feature of
chordates
Stomochord short diverticulum of the foregut
Diverticulum may be homologous to notochord of
chordates
- Suggesting the grouping of Hemichordates with Chordates is the
presence of pharyngeal slits and a blood vascular system
UROCHORDATES (or Tunicata)
- notochord is confined to the locomotor tail of the free-living larval stage
- enclosed in a beautifully transparent tunic
- filter feeders
- Ascidians undergo complete metamorphosis, resorb the tail and the
notochord before becoming sexually mature
- Larvaceans retain the tail and notohord throughout life and reproduce in
larval-like stage
- Thaliaceans have no larval stage, no notochord, and no tail.
1. ASCIDIANS sea squirts
- solitary and colonial
- larvae has a fleeting existence
- notochord is present
- nervous system (dorsal hollow nerve cord, several
ganglia & nerves)
- sensory vesicle (associated w/ brain that houses an
otolith & ocellus)
Otolith for stratoreception w/c stimulates nerve endings
Ocellus pigment-protected receptor cell
Metamorphosis: 3 adhesive papillae attach the larvae to permanent
substrate
Larvae
Larval mouth
Atriopore
Adult
Incurrent siphon (anterior)
Excurrent sipohon (posterior)
2. LARVACEANS appendicularians
- solitary, free-swimming planktons found in water
surface
- secrete mucopolysaccharides w/c surrounds organism
- has a tuniclike mass that is abandoned as it gets
clogged with debris from filter feeding
3. THALIACEANS the salps
- free-living individuals and colonial individuals in
alternate generations
- resembles adult ascidians (sea squirts)
- cylindrical in shape (excurrent and incurrent apertures
are on opposite sides of cylinder)
- differ from ascidians in having buccal and atrial
opening
- filter feeders
- no notochord
CEPHALOCHORDATA
-
Amphioxus means sharp at both ends or
Lancelet little spear a.k.a Branchiostoma
found near sandy beaches
burrow into sand w/ eel like movements
filter feeders
Locomotor Musculature and Skin
-
body is all trunk
exhibit metamerism from myomeres (lie under the
skin)
each myomere is separated from next by a
myoseptum (connective tissue partition that is the
origin and insertion of muscle)
utilize cutaneous respiration
Pharyngeal Slit
-
open in a fluid-filled cavity the atrium
pharynx does not serve a respiratory function
oxygen-rich water rapidly depletes to points where
there is a local oxygen debt
consists of muscular discs (contrast to chordates)
used to burrow (notochord is continuous to the tip of
the rostrum unlike chordate)
Notochord
Nervous System and Sense Organs
-
has a central nervous system consisting a hollow brain
dorsal nerve cord containing a central canal
both lined by ependyma (nonnervous supportive
membrane)
brain and cord surrounded by leptomeninx (single
connective tissue membrane)
only 2 brain subdivision (in contrast with craniates that
have 3)
notochord extends anterior to the brain (in craniates it
stops in the midbrain)
spinal nerve consists only of dorsal roots
have abundant chemoreceptors
have tactile receptors (elicit withdrawal)
most prominent is the light-sensitive ocelli (receptor
cell and caplike melanocyte)
Food Processing
filter feeders
vestibule (chamber for collecting seawater) w/c is
bounded laterally by an oral hood, caudally by a
perpendicular membranous velum
wheel organ covered by a sticky mucus retrieve
heavier food particles that miss the mouth
Coelom
-
not a prominent cavity in adults
becomes laterally compressed as additional
pharyngeal bars form in the elongating larvae
- larval coelom is prominent and metameric because it
forms from the metameric outpocketings of the
dorsolateral roof of the embryonic archenteron
Circulatory System
- amphioxus has no heart
- blood is a colorless cerum that lacks blood cells,
platelets, and other formed bodies
Elimination of Metabolic Waste
- amphioxus has no compact organ known as kidney
- cytopodocytes cells that collect metabolic wastes
(pedicels-glomerolus, microvilli)
Gonads
- mature gonads are visible throughout the body
- dioecious (ovary and testes do not develop in same
individual)
Amphioxus and the Craniates Contrasted
Amphioxus
Craniates
-cephalization
x
-paired sense organs
x
-notochord
/
-vertebral column
x
-pharyngeal slit
/
-dorsal hollow CNS
/
-craniate subdivisions
x
-metamerism
/
(extended to anterior tip of head)
-two layered skin
/
-arterial & venous channels
/
-heart
x
-coelomate
/ (restricted in adult)
ORIGIN OF CRANIATES
Ostracoderms majority of early vertebrate fossil
- posses hard bony parts
had no jaws
no paired fins
filter feeders
armored fishes because of the bony plates on the
skin
Cephalochordates
- have a notochord, pharyngeal slits, a dorsal hollow
central nervous system w/ brain and cord, metameric
bodywall musculature, two-layered skin, and arterial &
venous channels
- are deuterostomes coelomate
- filter feeders
THE AMMOCOETE: A VERTEBRATE LARVAE
- have a notochord the commences in the midbrain and
continous to the end of the body
- the dorsal hollow CNS develop the same manner as
that of anamphioxus
- several gills slits open to exterior
- body musculature is disposed as overlapping
myomeres that provide locomotion
- filter feeders
- CS is similar to that of an amphioxus but ammocoete
has a heart
- exhibit craniate features
