BIOL 335: Invertebrate Zoology

Fall 2008

Essay questions for Lecture Exam 2: Hydroskeletons, Rotifera, Acanthocephala, Mollusca, Annelida

Part II. Essay. 4 points. Describe the process of countercurrent exchange. Why is it adaptive?

· Characteristic of respiratory and excretory systems of higher invertebrates and vertebrates;

· Water and blood flow are antiparallel;

• Is adaptive because oxygen never reaches dynamic equilibrium between medium (water) and blood. There is always more oxygen diffusing into bloodstream at any point along the capillary bed of the gill.
  Part III. Essay. 4 points. Describe and contrast protonephridia, metanephridia and nephridia as excretory structures. Which phyla have each type of structure?

Protonephridia

• Characteristic of Platyhelminthes
• Flame cells possess tufts of cilia which induce water currents to move water through tubules and exit the body through nephridiopores.

Metanephridia

Molluscs have an open-ended, funnel-shaped metanephridia, and like other animals with metanephridia it filters the coelomic fluid by pulling it into the funnel's open end, the nephrostome. In molluscs, which have an open circulatory system, the largest body cavity is the hemocoel, filled with hemolymph (blood) that bathes the internal structures while collecting and circulating metabolic waste. The coelomic space and its fluids, which the metanephridia filter, are restricted to the small pericardial cavity surrounding the heart. There needs to be a way to get wastes from the hemolymph to the coelomic fluid so the metanephridia can do its job.

Nephridia

• Characteristic of Annelida (Also called metanephridia in Annelids)
• Tube-like structure similar to individual nephron, where secretion and reabsorption of nutrients occur through capillar network.
• Excretory wastes exit via nephridiopore.

Part IV. Essay. 4 points. Name and describe 3 criteria for a hydroskeleton. Describe a phylum which possesses this type of hydroskeleton. What type of musculature does this phylum have?

Shown here: Cross-section of Ascaris lumbricoides

Criteria for hydroskeleton:

• The presence of a cavity housing an incompressible fluid that transmits pressure changes uniformly in all directions;

• That this cavity be surrounded by a flexible outer body membrane, permitting deformations of the outer body wall to take place;

• That the volume of fluid in the cavity remain constant;

• That the animal be capable of forming temporary attachments to the substrate, if progressive locomotion is to occur on or within a substrate

• Found in Acanthocephala and Nematoda, shown here

Musculature in Nematoda includes longitudinal muscles only.