The Deuterostomes
Echinoderms and Chordates
Developmental Variations
During embryonic development, the
process of gastrulation forms the
digestive cavity.
 Gastrulation varies in the animal kingdom
 No gastrulation (placozoans)
 Incomplete gastrulation

◦ Forms a gastrovascular cavity with one
opening

Complete gastrulation
◦ Forms a digestive tract with 2 openings
Animals with Complete
Gastrulation
Complete gastrulation forms a true
digestive tract with 2 openings, the mouth
and the anus
 In protostome animals, the first opening
of the gastrula forms the mouth
 In deuterostome animals, the second
opening of the gastrula forms the mouth

Deuterostome Phyla

There are 3 phyla characterized by
deuterostome development
◦ Echinoderms
 Sea Cucumbers, Crinoids, Sea Stars, Brittle Stars
and Sea Urchins
◦ Hemichordates
 Acorn Worms and Pterobranchs
◦ Chordates
 Tunicates, Lancelets, Jawless fish and Vertebrates
Phylum Echinodermata


The echinoderms are unusual in the animal
kingdom in that they have a great deal in
common with the chordates and
hemichordates, but also differ in ways that
do not appear anywhere else in the animal
kingdom
It seems unlikely that their similarities with
other deuterostomes are the result of
convergent evolution, so it seems that
through the course of evolution the
echinoerms have lost a number of ancestral
characters retained in the other phyla
Characters retained:

Embryonic development & Gastrulation
◦ Triploblastic
◦ Complete digestive tract

Body Cavity
◦ Eucoelomate

Symmetry
◦ Bilateral during larval stages
Characters lost or diminished:
Segmentation
 Cephalization

◦ No head, brain or specialized sense organs
◦ Sensory apparatus are not centralized

Bilateral symmetry in the adult form
◦ The bilateral symmetry in the larvae goes
through a dramatic metamorphosis,
reorganizing the body into a Pentaradially
Symmetrical adult
Metamorphosis



The coelom forms as a U shape, which
pinches off into 3 distinct cavities, 2 of which
will become diminished
The digestive tract becomes reorganized,
with the formation of new mouth and anal
openings and a new orientation (mouth on
the left and anus on the right)
The left and right sides of the larvae
undergo differential development, with the
left side forming the oral surface and the
right side forming the aboral surface
Derived characters special to the
Echinoderms

Water-vascular system
◦ Consisting of a madroporite, a system of canals
(stone canal, ring canal, radial canals and lateral canals)
and sucker tipped tube feet

Dermal endoskeleton
◦ Consisting of calcified plates called ossicles and a
variety of spines and pedicellaria

Hemal system
◦ Poorly understood, it appears to function for
distribution of materials

Pentaradial symmetry
◦ Organs radiate from a central disc, characteristically
in a pattern of fives
Water Vascular System
Tube Feet
Crinoids (Sea Lilies)
https://www.youtube.com/watc
h?v=XR4NnmJfEXc
•
•
•
https://www.youtube.com/watch?v=cZcomBnN
KXg&feature=related
https://www.youtube.com/watch?v=hAdEfdyKB
HM
https://www.youtube.com/watch?v=jg_0UdbSRs
M&feature=relmfu
Sea Stars
• https://www.youtube.com/wa
tch?v=pSo30lRHaAw
• https://www.youtube.com/wa
tch?v=I8as-z-EShc
• https://www.youtube.com/wa
tch?v=-ardrFZuFkU
• https://www.youtube.
com/watch?v=qC89S
LOXIvs
• https://www.youtube.
com/watch?v=TioCre
e5axI
• https://www.youtube.
com/watch?v=Xm2m
F2IgLrA
Brittle Stars
• https://www.youtube.com/watch
?v=Myhp8ifW6ig
• https://www.youtube.com/watch
?v=Chs8Vk-6hoo
Sea Cucumbers
https://www.youtube.com/watch?
v=6mx0GDxfjQ4
• https://www.youtube.com/
watch?v=vsLBOkYLLeI
• https://www.youtube.com/
watch?v=iYSbLr-mVkM
Sea Urchins
• https://www.youtube.com/
watch?v=eGixkoZwEUs
• https://www.youtube.com/watch?v
=oljZbs5haaY
• https://www.youtube.com/watch?v
=WI7ZCK8aWoM
Echinoderms - Video

Sea Stars
o
o

https://www.youtube.com/watch?v=A100m5EpfFI
https://www.youtube.com/watch?v=rE8l-KFQlhY
Sea Urchins
◦ https://www.youtube.com/watch?v=nD7_0obUm0o
◦ https://www.youtube.com/watch?v=D3W4OCnHyCs

Sand Dollars
◦ https://www.youtube.com/watch?v=bSsgDhWpPq0
◦ https://www.youtube.com/watch?v=gLK71-vsi2E
Phylum Hemichordata
Hemichordates are deuterostomes, a
characteristic they share with both
echinoderms and chordates
 As the name suggests, hemichordates
have some derived characteristics that are
ancestral to the phylum chordata

◦ Dorsal, hollow nerve cord
◦ Gill slits in the pharynx

These traits are shared with all chordates,
but are absent in the echinoderms
Hemichordate Diversity

The phylum Hemichordata consists of
two classes
◦ The Acorn Worms (Enteropneusta)
◦ The Pterobranchs
The Acorn Worms and the Pterobranchs
are similar in body form, but vary with
regard to feeding structures
 Feeding structures in both are ciliated to
collect food particles and direct them to
the mouth

Acorn Worms – Feeding
The acorn worms have a muscular
proboscis used for food gathering located
anterior to the mouth opening
 The mouth is ventrally located
 A collar is associated with the mouth, and
is used both to direct food into the
mouth and to redirect particles too large
to ingest
 Acorn worms are free living and burrow
actively to search for food

Pterobranchs - Feeding
The proboscis itself is shield shaped
 The base of the proboscis bears several
tentacles which extend into the water to
create a food gathering surface for filter
feeding
 Pterobranchs move freely inside of a
collagenous tube, but are sessile in that
they are restricted to life within that tube

Gill Slits in the Pharynx
Hemichordates have one or more pairs of
gill slits that allow water from the
pharynx to pass out of the animal, rather
than being ingested
 Cilia maintain water flow through the gill
pouches
 Aside from providing a mechanism for
filtering and concentrating food particles,
the gill pouches create a surface for gas
exchange.

Central Nervous System



A point of emphasis in our discussion of the
evolution of the central nervous system has
been the presence in many invertebrate
phyla of ventral nerve cords
A ventral nerve cord persists in the
hemichordates, but is accompanied by a
dorsal nerve cord
The dorsal nerve cord is hollow because it is
formed by an invagination of ectodermal
embryonic tissue, as is the “spinal cord”
associated with the chordates
Phylum Chordata
The chordates are deuterostomes – they
possess a complete digestive tract with the
mouth forming from the 2nd opening of the
gastrula
 Chordates, like the hemichordates, have a
dorsal hollow nerve cord. Unlike
hemichordates they do not have a persistent
ventral nerve cord
 All chordates, at some time in their
development, have pairs of pharyngeal gill
slits

The Notochord
The distinguishing characteristic of phylum
Chordata is an endoskeleton centered
around the formation of a notochord
 The notochord is a rod of mesodermal
tissue located on the dorsal side of the
animal that extends almost the full length of
the body
 The notochord lies just ventral to the nerve
cord that forms the central nervous system

More notochord stuff
In the simplest of the chordates, the
notochord is a simple rod of tissue with a
fibrous sheath that provides some level of
longitudinal rigidity
 In other chordates, the notochord may
exhibit pronounced segmentation, and
may or may not become ossified
(impregnated with calcium, forming bone)

Ancestral and Derived Characters

Aside from the shared ancestral
characteristics that Chordates share with
their closest relatives (deuterostome
development, notochord, dorsal hollow nerve
cord & pharyngeal gill slits) the Chordates
share 2 derived characteristic only with
other Chordates:
◦ An endostyle (from which the thyroid gland is
derived)
◦ A post-anal tail
The Endostyle



The endostyle is a specialized organ
associated with the pharynx
In some of the invertebrate chordates the
endostyle functions largely as a filter-feeding
apparatus, secreting mucus for trapping food
particles in the pharynx
The endostyle persists in Chordates that do
not filter feed. The thyroid gland is derived
from the embryonic endostyle, and retains a
function related to feeding and metabolism
Chordate Diversity



The Chordates are a diverse phylum,
including both invertebrate and vertebrate
classes
The Protochordates are invertebrate filter
feeders, and consist of the Urochordates
(tunicates) and the Cephalochordates
(lancelets)
The Agnatha are the jawless fish. They are
the simplest bodied vertebrates, with a
cranium and a well developed tripartite brain
(three regions: forebrain, midbrain,
hindbrain)
Protochordates: Tunicates
Tunicate Anatomy



Tunicates are filter
feeders
Water enters the
mouth, filters
through the gill
slits in the
pharynx, and
passes out
through the atrial
siphon
Food passes
through a
complete
digestive tract
Protochordates: Lancelets
Lancelet Anatomy

Lancelets are
filter feeders
like the
tunicates, but
they have
more fully
developed
musculature,
notochord
and dorsal
nerve cord
Vertebrates with Jaws
The most familiar Chordates are the
Vertebrates.
 Aside from the Agnatha, the remaining
vertebrate Chordates have jaws derived
from the first 2 pairs of gill arches
 The ability to process food has evolved
independently in many different phyla.
Any level of processing has the potential
to improve digestion and thus increase
the chances of survival

Vertebrate Diversity - Fish


Agnatha – jawless fish (hagfish and lampreys)
Chondrichthyes – jawed fish with cartilage
skeletons (sharks and rays)
◦ The Chondrichthyes lack a swim bladder, and
maintain bouyancy through a large, oily liver

Osteichthyes – jawed fish with bony, calcified
skeletons (most familiar fish)
◦ The bony fish offset the density of their heavy
skeleton with an air filled swim bladder on the
dorsal side of their body cavity
Class Agnatha – Jawless Fish
Chondrichthyes – Cartilage Fish
Osteichthyes – Bony Fish
Poikilothermy vs. Homeothermy

Poikilothermic

◦ Temperature varies


“Cold Blooded”
Ectothermic
◦ Body temperature is a
function of the outside
environment

All invertebrates, as
well as fish, amphibians
and reptiles
Homeothermic
◦ Temperature is relatively
constant


“Warm Blooded”
Endothermic
◦ Body temperature
results from within
◦ Body heat derived from
metabolism

Dinosaurs
(theoretically), birds
and mammals
Vertebrate Diversity –
Poikilothermic Tetrapods

Class Amphibia
◦
◦
◦
◦

Includes frogs, toads, salamanders and newts.
Moist, vascular skin functions for gas exchange
The earliest terrestrial vertebrates
Amphibians are tied to moist environments by
the need to keep their skin and their eggs moist
Class Reptilia
◦ Includes lizards, turtles, crocodilians & snakes
◦ Dry, scaly skin is well adapted for life on land
◦ Shelled, amniotic egg frees reptiles from the need
to return to the water to lay eggs
Amphibians
Reptiles
Vertebrate Diversity –
Homeothermic Tetrapods

Class Mammalia
◦ Mammals are distinguished by a variety of skin
gland variations
 Hair glands
 Milk glands

Class Aves (Birds)
◦ Birds possess a number of adaptations unique to
the class Aves, including
 Feathers (derived from epidermal scale pouches)
 Bipedalism, with Forelimbs adapted for flight
 Sternum with a broad “keel” for pectoral muscle
attachment
Mammals
Birds (Class Aves)
Mammalian Diversity

Class Mammalia is divided into 3 groups
due to variations in reproductive behavior
and anatomy
◦ Monotremes
 Include the platypus and the echidna
◦ Marsupials
 Include the kangaroo, koala and opossum
◦ Placentals
 Include most familiar mammals, including humans
Monotremes

Monotremes have reproductive systems and
behaviors almost identical to reptiles. They are
oviparous (egg layers) and their eggs have leathery
shells
Marsupials


Marsupials are
viviparous. Their
young are born
live, but
immature and
poorly developed
Marsupial young
continue their
development
through
prolonged
lactation within a
protective pouch
Placental Mammals
Placental mammals
are live born after a
long gestation period
 The young are
nourished through
the placenta, which
imbeds in the wall of
the uterus and allows
exchange of
nutrients and waste

Marc Kirschner, Harvard:
How the chordate got its cord
https://www.youtube.com/watch?v=232wt
CuLsoI
 https://www.youtube.com/watch?v=Y0xfp
Gdh1_Y
 https://www.youtube.com/watch?v=I1yqk
SuTwrU
