Lecture 13 - Bilateria and Placozoa

Question 1

Placozoa: what are they, where do they live, and what do eat?

Answer 1

Flat animals that:

• Glide on cilia and can change shape like amoeba
• Contains four types of cells
• Can do fission or budding
• Have no neurons, muscles or digestive system but have genes that are used in defining dorsal/ventral axis, although they have no mouth – ‘ventral’ surface produces a digestive enzyme
• Lack “basal lamina” – part of extracellular matrix on which other cells sit
• Have their position in animal phylogeny disputed

Found in coral reefs, and mangrove swamps in highly variable genotypes around the world but rarely in strong tidal flow or sand

Algae? Predators deterred by venomous ‘silver balls’ (‘crystal cell’)

Question 2

Bilaria: why did species evolve in this way?

Answer 2

• 99% of described species are bilaterians
• 34/38 living animal phyla
• Far more complex structures, more diverse morphology and ecology than radially symmetrical organisms
• That third embryological tissue layer (mesoderm) used to make interesting things

Question 3

Diploblastic: what does it mean and what are the features of the animals that are diploblastic?

Answer 3

Having a body derived from only two embryonic cell layers (ectoderm and endoderm, but no mesoderm), as in sponges and coelenterates

Water provides support (hydrostatic skeleton – this principle is also found in some triploblastic animals such as earthworms, and sea anemones)

Question 4

Protostomes and deuterostomes: the classification struggle

Answer 4

Until recently, Bilateria was divided into two groups:

• Protostomes – mouth develops first (‘new mouth’)
• Deuterostomes – anus develops first (‘old mouth’)

Which comes first, anus or mouth was the criterion

Turns out some ‘protostomes’ develop their anus first, or don’t have an anus…

Question 5

The coelum

Answer 5

The cavity in the body often lined by an epithelium derived from the mesoderm (but not all – molluscs)

• Used in a variety of ways, primarily for making a through gut and suspending internal organs
• Not all triploblasts have a coelom – platyhelminth worms do not (‘acoelomate’)
• A mixture of animals (rotifers, nematodes etc) have a cavity that is not strictly a coelom (‘pseudocoelomate’) – this is probably a derived condition, explained by ecological pressures

Question 6

Ecdysozoa: what are they, what are their features, and what animals are part of this clade?

Answer 6

A group of bilateral animals that moult with a tough, but flexible exoskeleton that protects them from water loss, predators, and other aspects of the external environment. All members of this superphylum periodically moult or shed their cuticle as they grow

Arthropods, Tardigrades, Onychophorans (that share segmentation and appendages), Nematodes, Nematomorphs, Priapulids, Kinorhynchs and Loriciferans

Question 7

Lophotrochozoa: what are they, what are the key features, and what animals are part of this clade?

Answer 7

A clade of bilateral animals that have triploblastic embryos (ectoderm, endoderm, and mesoderm)
with protostome characteristics and may be either coelomate or pseudocoelomate (depending on phylum) with true tissues, organs and organ systems: integumentary system. digestive system.

Annelids, Molluscs, Bryozoans, Brachiopods, Platyhelminthes, and other animals that descended from the common ancestor of these organisms

Question 8

Deuterostomia: what are they, what are the key features, and what animals are part of this clade?

Answer 8

A clade of bilateral animals that have the anus formed before the mouth, the mesoderm formed from pockets of the gut, and radial, regulative, and indeterminate cleavage

Echinodermata, Chordata, Chaetognatha, and Brachiopoda

Question 9

Platyhelminthes: what are they, what are the key features, and what animals are part of this clade?

Answer 9

Bilateral organisms with internal space (digestive cavity); no coelom, beginning of organ-level specialization

• Regenerative ability
• Two nerve cords in most developed, plus eyespots
• Many are parasitic
• Mainly aquatic, but can be terrestrial

Taenia (Tapeworms), Fasciola (Liver fluke), Taenia saginata (Beef tapeworm), Echinococcus granulosus (the dog tapeworm), Planaria (freshwater flatworm), Opistorchis.

Question 10

Mollusca: what are they, what are the key features, and what animals are part of this clade?

Answer 10

Bilaterally symmetrical organisms that have their bodies comprised of a head, visceral mass (holds the bulk of the digestive, reproductive, excretory, and respiratory systems), muscular foot, and mantle

*They are mostly found in marine and freshwater
* Organ system level of organization.
* Body has a cavity
* May have tentacles and compound eyes
* May have the body covered by a calcareous shell

Gastropods, Bivalves, Cephalopods (Scavengers and predators – fast, intelligent, solitary, can be highly visual, able to manipulate objects), etc

Question 11

Mollusca ancestry

Answer 11

The radula is a key feature of molluscs. Lost in some lineages, it becomes the ‘beak’ in cephalopods.

Tentacles and arms in cephalopods are adapted from the foot

Question 12

Triploblastic: what does it mean and what are the features of the animals that are triploblastic?

Answer 12

Contain three primary layers: ectoderm, endoderm and mesoderm

Form tissues and organ systems

Question 13

Ectoderm

Answer 13

It is the outermost layer that forms skin, nails, hair, etc

Question 14

Endoderm

Answer 14

Endoderm - It is the innermost layer that forms the stomach, colon, urinary bladder, etc.

Question 15

Mesoderm

Answer 15

Mesoderm - It is the middle layer between ectoderm and endoderm which forms bones, cartilage, etc.