This exam is worth 70 points.

Shorter Answer (30/70 points). Provide short statements, diagrams, figures, or lists.

1. (6 pts) Which hypothesis seems most likely to explain the origins of Porifera: Symbiotic, Cellularization, or Colonial? Which protozoan group is the likely ancestor of Sponges, and why?

Colonial: some colonial protozoans resemble blastula stage of early metazoans like porifera. Early metazoans lack multinucleate stage required by Cellularization hypothesis, and totipotentency of Porifera makes Symbiotic unnecessary.

Choanoflagellates are colonial and have a collar cell that resembles choanocytes of Porifera. Molecular phylogenetic reconstructions show close relationship between Choanoflagellates and Porifera.

2. (6 pts) How does the Anthozoan polyp differ from Hydrozoan polyp? What characteristics do the Anthozoan polyp and Scyphozoan medusa share?

Anthozoan polyp: septate gut, stomodeum with siphonoglyphs, gastrodermal gonads, cellular mesoglea. Hydrozoan polyp is simple and lack other features.

Both Anthozoan polyp and Scyphozoan medusa have gastrodermal gonads, divided gut, thick cellular mesoglea.

3. (6 pts) Describe the developmental processes that set up the bilateral body plan (i.e. ’worms’). What external and internal features are associated with a bilateral body plan?

After cleavage, blastula formation and gastrulation leads to major body axis. Features include a single mirror image plane of symmetry, longitudinal and circular muscles, long. nerve cords, cephalization.

4. (6 pts) Describe a typical trematode life cycle. How does it differ from the life cycle of a parasitic nematode?

Trematode: two or more hosts, asexual reproduction of larval stages in an invertebrate (snail) intermediate host, sexual reproduction of adult stage in a vertebrate host. Parasite nematodes have a single host, site of sexual reproduction, lack of asexual reproduction, and free-living larval stage in the soil. Some parasite nematodes infect plants.

5. (6 pts) Phyla Rotifer and Nematoda are both protostomes. What 3 developmental characteristics do protostomes share? To which of the two clades of protostomes do these two phyla belong to, and what characteristic distinguishes the two clades?

Protostomes: spiral cleaveage, mosaic development, mouth forms from first opening at gastrulation, pseudocoelomates.

Rotifers are Platyzoa (Lophotrochozoa) and do not molt. Nematodes are Ecdysozoa and molt their cuticle.
Essay questions. Answer 2 total. ANSWER 1. THEN ANSWER 2. OR 3.

1. REQUIRED (20 pts) What characteristics are shared between Ctenophores and Cnidarians, and in what ways do these groups differ? What evidence suggests that the Ctenophores might be triploblastic and related to bilateral animals? What does the phylogeny in Fig 1 suggest about the ancestor of bilateral animals, and the origin of Ctenophores?

Ctenophores share with Cnidarias: biradial symmetry (a modified radial symmetry), mesoglea, nerve net, gastrovascular canals.

Ctenophores differ from Cnidarians: multiciliate cells, gastrulation is invagination or epiboly (not ingression), mouth and anus, muscles in mesoglea (triploblastic?), colloblasts (vs. cnidocytes), apical sense organ, cydippid larva and direct life cycle (no polyp), no asexual reproduction.

One difference is the smooth muscle cells in the mesoglea of Ctenophores. These appear to be of mesoderm origin. This might suggest that Ctenophores arose from a bilateral triploblastic ancestor. However, Fig 1 shows that Ctenophores did not originate from a common ancestor with bilateral animals. Cnidarians seem to have a more recent common ancestor with bilateral animals than Ctenophores.

ANSWER 2. OR 3.

2. (20 pts) In the Cnidarians, the Class with the simplest life-cycle has the most complex polyp form, and this complicates understanding their evolutionary relationships. Describe two hypotheses for the relationships among these classes. Which Class and which life cycle do you believe is ancestral? Why?

The Polyp-first hypothesis (Medusozoa hypothesis): states that the polyp-only life cycle arose first, and the medusa was added to the life cycle later. Classes with medusa (Hydrozoa, Scyphozoa and Cubozoa) form a monophyletic clade called Medusozoa.

The alternative is the Medusa first hypothesis: That the ancestral life cycle possessed medusa only, and the Trachymedusa (Trachyline hydrozoa) might represent the ancestral state.

Supporting the Polyp-first hypothesis: DNA trees were slightly more parsimonious for trees consistent with this hypothesis. This evidence might be viewed as more „conclusive” because characters are discrete and homology is more certain. This requires that the complex polyp traits of Anthozoans are apomorphies (not-shared derived traits), or that the polyp traits of the other classe evolved by simplification. One difficulty is to imagine the transition of sexual reproduction from polyp to medusa.

Supporting the Medusa-first hypothesis: Hydrozoan polyp is simpler morphologically. Similarities between Anthozoan polyp and Scyphozoan medusa suggest relationship.

3. (20 pts) What are some similarities and differences between the free-living Platyhelminthe and Nemertean worms? Do you think they are closely or distantly related and why? Which characteristics suggest a close relationship? A distant relationship?

Are Platyhelminthes, Nemetereans, both, or neither, likely to be the first bilateral triploblast? Explain your answer.

Similarities: D-V flattened body, solid „acoelomate” condition, ciliated epidermis, protonephridia, rhabdites, longitudinal and circular and transverse muscles, ladder-like nervous system

Differences: closed circulation, rhynchocoel and proboscis distinct from gut, circum-rhynchocoel ganglia, mouth and anus (Nemerteans)

Distant relationship is suggested by rhyncocoel and closed circulatory system, assuming that these represent coelom remnants in Nemerteans. DNA sequence phylogenies do not indicate a close relationship.

Neither. One could argue that Nemerteans are derived group from coelomate ancestors, due to the rhynchocoel and circulatory system as coelom remnants, and hence they had a triploblast ancestor. Platyhelminths are the simplest bilateral triploblasts, but some DNA phylogenetic evidence suggests that they are not basal among triploblasts. A simple Platyhelminth-like animal called Acoela consistently appears in phylogenetic reconstruction as basal to triploplasts.