ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Chapter Outline		AP BIOLOGY: Chapter Thirty-Nine Review Answers 1. The two subkingdoms of animals are Parazoa, which are asymmetrical and have no tissues or organs, and Eumetazoa, which are symmetrical and have tissues organized into organs and organ systems. 2. Both of the animal subkingdoms evolved from choanoflagellates. The general characteristic that divides the lower and higher invertebrates is that the lower have less complex tissue organization. The four major phyla of the "lower" invertebrates are Porifera, Cnidaria, Platyhelminthes, and Nematoda. 3. In a sponge, food particles stick to microfibrils connecting collar strands, which serve as a food sieve. It is externally digested directly by collar cells or by amoeboid cells in the spongocoel. 4. The primary stages in sponge reproduction are as follows: gametes are shed into the water for external fertilization; initial development occurs within the body of the adult; when released, the larvae have many external flagellated choanocytes that are free swimming. After the planktonic stage, they settle onto a suitable substrate, and turn inside out so that the choanocytes of the adult are on the inside. 5. A cnidarian body is a gelatinous matrix with distinct tissue differentiation. The two body forms present are the polyp, a cylindrical diploid form that is attached to a substrate and is solitary or colonial, whose mouth points upward, and who may build an external skeleton diploid; and the medusa, an umbrella-shaped, free-floating organism, whose mouth points downward. A planula is a free-swimming, multicellular, ciliated cnidarian larva derived from fusion of gametes. 6. Digestion occurs within the gastrovascular cavity in cnidarians and is extracellular. The chemical means of this digestion are enzymes released into the gastrovascular cavity that break the food into bits that are then phagocytized by the cells lining the gut. The indigestible particles go out through the same opening that they entered. 7. Cnidarians capture their food with harpoonlike structures called nematocysts, which they can discharge under a tremendous amount of pressure to puncture even the exoskeletons of arthropods. Pressure builds up within the cnidocyte-the cell in which the undischarged nematocyst sits tightly coiled-and when pressure is sufficient, the nematocyst is forced out, everting in the process. Many times the nematocyst is accompanied by an enzymatic discharge, making the puncture wound particularly painful, thus, the name "stinging nettles." 8. Platyhelminthes' body plan is acoelomate. They are dorsoventrally flattened and have a distinct head at the anterior end. Most are parasitic, although some are free-living. They move through ciliated epithelial cells on their ventral surface. The digestive system is with the gut branched, with only one opening. Food is partly digested extracellularly, and phagocytic cells line the gut. Some parasites absorb food through the body wall. 9. The sensory structures are chemical-sensing tentacles and light-sensitive eyespots. These structures orient the animal to food and away from the light. With regard to sexual reproduction, the individuals are hermaphroditic, and each deposits sperm in the copulatory sac of the mate, laid in cocoons in the free-living forms. The parasitic forms may have several larval forms. Some can regenerate whole individuals when divided. 10. Flukes ingest food through the mouth. Two or more hosts are needed to complete an entire life cycle. The stages of the life cycle are (1) the egg hatches into a free-swimming miracidium, (2) when digested by a snail it transforms into a sporocyst, (3) from which rediae hatch, (4) these grow into cercariae which are liberated into the water; (5) these attach to fish and transform into metacercariae encysting in muscles, and (6) a mammal eats the fish and cysts dissolve, releasing the young flukes, which then lay eggs inside the mammalian host. 11. Flukes possess a digestive tract, tapeworms do not; tapeworms absorb their nutrients directly over the body wall. The tapeworm is composed of a head provided with suckers, hooks, or both, a neck region, and an extended region of repeating segments which become reproductively active with distance from the head. These segments can release gametes as well as become detached and pass out with the feces of the host organism. 12. Nematodes are pseudocoelomate animals. The digestive tract of a typical nematode has sensory organs at the anterior end and a mouth equipped with piercing stylets. The muscular pharynx sucks food inside the mouth, and food then continues through the digestive tract and is eliminated through the anus. The nematodes are structurally unique because they completely lack cilia and flagella, even in the sperm and excretory organs. They are damaging to humans because many are parasites in the digestive tract, causing anemias, damaging tissues, and obstructing circulatory vessels. 13. Rotifers are pseudocoelomate aquatic animals that use a crown of cilia to sweep particulate food materials into a muscular grinding area. Rotifers sexes are separate and sexual reproduction occurs, but parthenogenesis is also exhibited by some groups, in which unfertilized eggs develop directly into females.