LECTURE 05 - Bilateria

Question 1

Describe the axial patterning in development of Hydrozoa.

Answer 1

• The fundamental asymmetry in the fertilized egg is the animal/vegetal axis
• The animal pole is defined by the site of polar body formation, where the oocyte is in contact with the germinal epithelium
• The animal pole is the site of the unipolar first cleavage furrow and the site of gastrulation (blastopore)

Question 2

In Clytia, what does the animal pole of the egg mark?

Answer 2

The animal pole of the egg marks the site of cell ingression at gastrulation and the future oral pole of the embryo and planula larva

Question 3

Where is the site of gastrulation in Cnidaria?

Answer 3

• Site of gastrulation remains at animal pole
• Blastopore becomes mouth and hence establishes oral-aboral axis
• Cells in this region give rise to gastrodermal cells of gastric cavity and to derivatives such as nerve cells of endodermal nerve net

Question 4

Where is the site of gastrulation for Bilateria? What happens at the animal pole?

Answer 4

• Site of gastrulation switches to vegetal pole
• Mouth remains at oral pole
• Mouth formation and endomesoderm formation have become separated
• Developmental systems of head and body can evolve independently
• Animal pole cells form anterior head and brain of adult
• Mouth forms from oral ectoderm independent of blastopore
• Endomesoderm splits into endoderm (gut) and mesoderm (muscle, circulatory system, nephridia, gonads, etc.)

Question 5

What does Bilateria split into?

Answer 5

Protostomia
Deuterostomia
And possibly a third clade, Xenacoelomorpha

Question 6

What are the two main groups of Protostomia?

Answer 6

• Spiralia
• Ecdysozoa
• and probably a third clade, Chaetognatha

Question 7

Describe what changes differentiate Protostomia from Bilateria.

Answer 7

Protostomia
- Mouth arises at site of gastrulation between animal pole and equator
- Anus forms independently of site of gastrulation

Question 8

Describe what changes differentiate Deuterostomia from Bilateria.

Answer 8

Deuterostomia
- Blastopore at site of gastrulation becomes anus

Question 9

What phyla are deuterostomes and what differentiates them from each other?

Answer 9

• Ambulacraria
  
  • Mesoderm formed from vegetal pole
• Chordata
  
  • Mesoderm formed from equatorial region

Question 10

How are complex body plans of Bilateria made possible?

Answer 10

Complex body plans of Bilateria are made possible by the separation of tissue-forming regions in the early embryo

Question 11

Describe protostomes and deuterostomes

Answer 11

• The coelomate Bilateria fall into two large clades, Protostomia and Deuterostomia
• Deuterostomia is the smaller group, comprising echinoderms, chordates, and a few others
• Protostomia includes the bulk of animal diversity
• The distinction between the two was made by comparative anatomists and embryologists a century ago and has been strongly supported by more recent molecular phylogenetic analyses

Question 12

How many stages did the stem Metazoa pass through?

Answer 12

3
- acoelomate
- pseudocoelomate
- coelomate
(in that order)

Question 13

In the current consensus view, what is the sister group of all other Bilateria?

Answer 13

Acoel flatworms (previously included in Platyhelminthes - but are now interpreted as a separate clade) are the sister group of all other Bilateria

Question 14

What is acoela?

Answer 14

• Acoela is a group of bilaterally symmetric animals with an apparent morphological simplicity: they lack body cavities, corporal segmentation, circulatory and respiratory systems, nephridia or protonephridia and larval stages, and their digestive system only has one opening to the exterior

Question 15

How many members does Acoela have? What are they?

Answer 15

3
- Acoela
- Nemertodermata
- Xenoturbella

Question 16

Describe the anatomy of Acoela

Answer 16

• Acoels are acoelomate, the space between gut and body wall being filled with parenchymal cells that occasionally contain chordoid vacuoles and the insunk bodies of epidermal and gland cells
• The name “acoel” comes from their lack of a cavity in the gut, which is typically a solid syncytium - that is, they lack even a primary body cavity
• Epidermis lacks a basal lamina

Question 17

Describe what Nemertodermatida are.

Answer 17

• Minute obscure interstitial marine worms
• They share some characters with Acoela, notably a distinctive ciliary rootlet system and a peculiar duet spiral cleavage during the early stages of development
• Nematoderms have a gut lumen, although it is narrow and rather occluded, and as in acoels lacks an anus

Question 18

Describe Xenoturbella

Answer 18

• Seafloor, off coast of Sweden, Scotland and Iceland; can be collected regularly only off west coast of Sweden at 50 - 150 m depth
• Simple sac, 1-3 cm long, ventral mouth, ciliated externally
• Body wall with circular (outside) and longitudinal muscles
• Can burrow or glide on surface of substrate
• Diet mysterious (probably mollusk larvae)
• No brain; diffuse nervous system with intra-epidermal nerve net
• No through-gut
• No excretory system
• No circulatory system
• No discrete reproductive system (but does have eggs and typical sperm)
• More or less direct development with short swimming stage of uniformly ciliated larva

Question 19

What are Chaetognatha?

Answer 19

• Elongate bilaterians of the marine plankton
• 5-120 mm in length
• Known from early Cambrian; putative ancestral group (protoconodonts) from earliest Cambrian
• Extremely abundant in all oceans
• All are predators that eat zooplankton, especially copepods; also fish larvae

Question 20

Who are the dominant predators of the photic zone of the ocean?

Answer 20

Chaetognaths and cnidarians

Question 21

What kind of predators are Chaetognaths?

Answer 21

Chaetognaths are ambush predators that perceive their prey through hydromechanical signals
- They eat anything that is large enough to be perceived and small enough to be swallowed

Question 22

Describe the anatomy of Chaetognaths

Answer 22

• Body bears lateral fins used in swimming and anterior paired groups of grasping spines used in prey capture
• Spines are sharp curved chitinous structures formed by epidermis, operated by complex musculature
• Body usually turgid
• Fold of body wall can be drawn over spines as hood
• Body wall with very thin cuticle
• Coelom in three compartments, divided by septa behind head and behind anus
• Through-gut a straight tube
• Nervous system of ganglia in head connected by circumenteric commissures to a large ventral trunk ganglion
• No circulatory, respiratory or excretory systems
• Coelomic fluid is circulated by cilia of lining

Question 23

Explain Chaetognath’s reproduction and development

Answer 23

• Chaetognaths are hermaphroditic; ovaries mature after tail coelom is filled with sperm
• Outcrossed with exchange of spermatophores
• Fertilization is extraordinary and involves the transport of sperm to ovule by an accessory cell
• Early cleavage total, equal and more or less radial (i.e., deuterostome-like)
• Blastopore forms anus (also deuterostome-like) and opening opposite forms mouth; both then close; and subsequently re-open
• Coelem and mesoderm formation appear to be unusual and not clearly either protostome or deuterostome