Comparative Anatomy Chapter 2

Blastocyst

early stage of embryo described as a “ball of cells”

Cleavage

division of cells from the egg/zygote

Radial cleavage

cells are aligned as they divide

Spiral cleavage

cells are offset from each other

Gastrulation

cells growing inward (invagination); these indented cells become the gut and continue until they reaches the other wall, and then they usually break through and form a second opening

Blastopore

first opening into the indentation of gastrulation

What are the layers that form after gastrulation?

1) Ectoderm

2) Endoderm

3) Mesoderm

4) Coelom

5) Schizocoelom

6) Enterocoelom

Ectoderm

Outer layer

Endoderm

inner layer that forms the lining of the gut

Mesoderm

middle layer between the ectoderm and endoderm

Coelom

Internal body cavity

Schizocoelom

body cavity made from solid mass of mesodermal cells splitting

Enterocoelom

body cavity forming from the outpocketing of the gut that pinches off

What are the characteristics of the protosomes?

What are the characteristics of the deuterosomes?

What is a chordate?

What are the characteristics of chordates?

What is a notochord?

A slender rod that forms from the mesoderm that is dorsal to the coelom and ventral of the nerve cord; a hydrostatic organ

Hydrostatic organ

Outer fibrous sheath encloses incompressible fluid that prevents axial compression

What is the structure of notochords?

Composed of a core of cells and fluid encase in a tough fibrous tissue sheath

What is the function of notochords?

Essential for vertebrate development as it signals the formation of the nerve cord.

Pharyngeal slits

Used for filter feeding

Dorsal Hollow Nerve Cord

Endostyle

Glandular groove in the floor of the pharynx used in filter feeding; involved in iodine metabolism

Thyroid gland

Produces thyroid hormones and calcitonin; involved in iodine metabolism

Postanal Tail

Myomeres

Blocks of serially repeated muscles that run the length of the adult body and tail

How do acorn worms (Enteropneusta) eat?

By filter feeding with cilia and mucus

Which group is most closely related to vertebrates?

Urochordates

What is an enterocoelom?

Body cavity formed from gut outpocketing that pinches off

The notochord eventually develops into the spinal cord.

False

What structure do hemichordates and chordates share

Pharyngeal slits

What group includes sea stars?

Echinodermata

What group includes sea urchins?

Echinodermata

What group includes the sand dollar?

Echinodermata

What group includes the sea cucumber?

Echinodermata

What group includes the sea lily?

Echinodermata

What are the characteristics of phylum Echinodermata?

1) An Unsegmented adult body

2) Endoskeleton of calcium carbonate

3) No head or brain but a nerve ring

4) Pentaradial symmetry

What are Protochordates?

marine animals that use cilia and mucus to filter feed

Are protochordates a natural (monophyletic) group?

No

What environment do all protochordates live in?

Marine

What is the larval lifestyle of protochordates?

Pelagic and planktonic

What does “pelagic” mean?

Living in open water, not on the bottom

What does “planktonic” mean?

Drifts with currents, limited self-movement

Where do protochordate adults usually live?

Benthic (on or in the substrate)

How can protochordate adults interact with the substrate?

Burrow into it or attach to it (sessile)

What does “sessile” mean?

Permanently attached, non-moving

Are protochordates all monoecious?

No

What reproductive strategies occur in protochordates?

Dioecious or monoecious

Why are protochordates considered “not a natural group”?

They don’t share a single exclusive common ancestor

What shared traits cause protochordates to be grouped together despite not being natural?

Marine habitat, filter feeding, similar larval stages

Why is larval form important in protochordate classification?

Reveals evolutionary relationships hidden in adults

What does protochordate diversity suggest about early chordate evolution?

Multiple lineages experimenting with similar lifestyles

What are the 3 types of Protochordates?

Hemichordata, Urochordata, and Cephalochordata

What group does phylum hemichordata fall under?

Protochordates

What is the general body type of hemichordates?

Half chordate Marine benthic worms

Are hemichordates marine or terrestrial?

Marine

What are the two hemichordate classes?

Enteropneusta and Pterobranchia

What is the common name for Enteropneusta?

Acorn worms

Which chordate characteristic do hemichordates have?

Pharyngeal slits

Where is the hemichordate dorsal nerve cord located?

The Collar region

Is the hemichordate nerve cord a true chordate nerve cord?

No

What is the larval form of hemichordates called?

Tornaria larva

Tornaria larva resembles which echinoderm larva?

Auricularia

Why is larval similarity important?

Indicates close evolutionary relationship

summary of Hemichordata?

Marine deuterostomes with pharyngeal slits and echinoderm-like larvae bridging echinoderms and chordates

What role do hemichordates play in chordate evolution?

It acts as a link between echinoderms and chordates

Why aren’t hemichordates classified as chordates?

Lack a true notochord and true dorsal hollow nerve cord

summary of protochordates?

Marine filter feeders with planktonic larvae, benthic adults, and no single evolutionary origin

Which are more closely related: echinoderms and chordates OR arthropods and chordates?

Echinoderms and chordates

Are hemichordates closer to echinoderms or arthropods?

Echinoderms

Why are echinoderms close to chordates?

Both are deuterostomes

Which are closer: hemichordates and echinoderms OR hemichordates and arthropods?

Hemichordates and echinoderms

Are hemichordates closer to chordates or arthropods?

Chordates

Are cephalochordates closer to vertebrates than urochordates?

No

Closest living relatives of vertebrates?

Urchochordates

If two animals share pharyngeal slits, are they closely related?

Not necessarily—trait may be ancestral

Are sponges closely related to chordates?

No

Which is more distantly related to chordates: arthropods or echinoderms?

Arthropods

Are acorn worms pelagic or benthic?

Benthic

What type of substrate do acorn worms live in?

Sand or mud

What feeding-related structures do enteropneusts have?

Pharyngeal (gill) slits

What depth range do enteropneusts occupy?

Tidal zone to deep sea

summary of Enteropneusta?

Solitary, benthic hemichordates with pharyngeal slits living in marine sand or mud

Why aren’t enteropneusts considered true chordates despite slits?

No true notochord or dorsal hollow nerve cord

Which class of Hemichordata is more mobile?

Enteropneusta

How can you distinguish Enteropneusta from Pterobranchia?

Enteropneusta are solitary and burrowing

How many pharyngeal slits can enteropneusts have?

1–200

What is the size range of acorn worms?

2 cm to 2.5 m

summary of Pterobranchia?

Colonial hemichordates that live in tubes and likely evolved from acorn worms

Key difference between Pterobranchia and Enteropneusta?

Colonial tube-dwellers vs solitary burrowers

Pterobranchia are thought to have evolved from which group?

Enteropneusta (acorn worms)

Pterobranchia belongs to which phylum?

Hemichordata

Name one genus of Pterobranchia.

Rhabdopleura

What type of lifestyle do Pterobranchia have?

Colonial

Where do pterobranchs live?

In self-made tubes

Are pterobranchs free-living or attached?

Attached (sessile)

Which group is colonial, tube-dwelling, and hemichordate?

Pterobranchia

Which group is solitary, burrowing, and hemichordate?

Enteropneusta

Lifestyle: Enteropneusta vs Pterobranchia

Enteropneusta = solitary; Pterobranchia = colonial