Question

Subject: Annelids & Molluscs

Question: Sketch a cladogram showing evolutionary relationships between the groups we studied and identify apomorphies that distinquish the groups

**NOTE**

Do not write in cursive, thanks!

Answer 1

Phylum Mollusca

• Molluscs include squids, octopuses, clams, scallops, oysters, snails, and slugs. They are bilateral, coelomate, protostomes.
• All mollusks have a visceral mass, a mantle, and a foot. The visceral mass contains the digestive, excretory, and reproductive organs. The mantle is a covering. It may secrete a shell. The foot is muscular and is used for locomotion, attachment, and/or food capture.
• The mantle and foot can be seen in the photograph below. The visceral mass is underneath the gill.
• There may be a radula, a structure that resembles a tongue but contains hard plates and is often used for scraping food.
• The coelom is reduced and limited to the region near the heart.
• Most mollusks have an open circulatory system but cephalopods (squids, octopus) have a closed circulatory system.
• The blood pigment of mollusks is hemocyanin, not hemoglobin.
• Bivalves have three pairs of ganglia but do not have a brain.
• Most mollusks have separate sexes but most snails (gastropods) are hermaphrodites. Some marine mollusks have a ciliated larval form called a trochophore.

Phylum Annelida

• Annelids are bilateral, coelomate protostomes.
• The coelom is partitioned by septa (crosswalls).
• The fluid-filled coelom acts as a hydrostatic skeleton. When the circular muscles that surround each segment contract, the segment becomes thinner and longer. When the longitudinal muscles that extend from one end of the segment to the other contract, the segment becomes shorter but thicker.
• Because muscles can only contract and cannot lengthen, other muscles are used to lengthen them. In annelids, when circular muscles contract to lengthen the segment, the longitudinal muscles are lengthened. When the longitudinal muscles contract to make the segment shorter and thicker, the circular muscles become lengthened.
• Setae are bristles on the skin that anchor or help move the animal. Movement occurs when waves of contraction of longitudinal muscles cause a "bulge" to progress from the anterior end to the posterior end.
• Annelids exhibit specialization of the digestive tract. Some of these structures are the pharynx, crop, gizzard, intestine, and accessory glands.
• Annelids have a closed circulatory system.
• A pair of cerebral ganglia function as a simple brain. A  ventral nerve cord extends the length of the animal and connects to a pair of fused ganglia (mass of nervous tissue) in each segment. The ganglia within each segment function to coordinate muscle contractions.
  

A close evolutionary relationship between annelids and mollusks is suggested by the presence of a trocophore in both phyla as well as by molecular sequence comparisons.

Cladograms are evolutionary tree diagrams that show relationships based on synapomorphies (shared-derived characters).