POSSIBLE ANSWERS FOR TAKE-HOME ASSIGNMENT # 3 INVERTEBRATE ZOOLOGY 1999
DR. W. CRONE (303 FTZ, 629-7439, cronewil@hvcc.edu)
Topics: Molluscs, Annelids, Arthropods
A model mollusc highlights major features of the phylum, even if such a creature never existed. A ventral muscular head-foot underlies a visceral mass containing"feeding and breeding" (digestive and reproductive) organs. The visceral mass, in turn, is draped by a fleshy mantle dorsally, which then produces an incomplete exoskeleton of a shell. Between the mantle and the visceral mass posteriorly is a mantle cavity, in which lie specialized ciliated gills or ctenidia. None of the current classes of mollusc follows this model exactly, but the chitons (Class Polyplacophora) come close. The ventral head-foot, a visceral mass with a posterior mantle cavity, and overlying mantle and shells appear at first glance to be close to this model. There are some differences, though. The mantle is very large and overhangs around the foot. The mantle cavity with the gills is also very extensive around the foot, instead of just posteriorly. Finally, the chiton shell is made of eight plates, instead of one large unit. In summary, the"model mollusc" is a useful concept to highlight the common features of the molluscs, even if it does not fully explain all of the features seen in today's molluscan species.
(Sources: Week 8 lecture notes and handouts; Hickman et al., 7th ed., Ch. 21, pp. 451, 453-455)
Metamerism is a situation of serially repeated body parts or metameres making up the body of an organism. Having separated , relatively identical units making up the length of a body has three major advantages:
Metamerism has a definite influence on the life style and shape of a typical polychaete such as a clam worm (genus Nereis). The multiple body units, each with their own pair of parapodial projections, allow for a wriggly movement as it seeks out prey. This reflects bending of some body segments in one direction, as vs. bending others in another direction. A well-developed head, with strong jaws, represents the specialization of several head segments. The parapodia, in addition to providing a"rowing," serpentine motion, also add surface area for underwater respiration for the clamworm, who does not have specialized gills otherwise. Overall, therefore, metamerism has a definite effect on the shape and lifestyle of the clamworm.
(Sources: Week 9 lecture notes and handouts; Hickman et al., 7th ed., Ch. 22, pp. 474, 477, 478)
3. The exoskeleton of an arthropod covers it in an armor-like fashion. Despite this, how does an arthropod exoskeleton permits movement and growth?
The exoskeleton of an arthopod is a hardened cuticle secreted by the epidermis. In order for movement, some areas of the cuticle are softer and allow hinging. Many individual muscles are attached to the outer shell in order to create movement. In the appendages, these muscles are set up within multiple hinge joints, as these allow a wide range of motions but does not require a ball-and-socket design unable to be achieved with an exoskeleton. In order for growth, the hardened exoskeleton must be shed for the animal to enlarge. An arthropod will be vulnerable when the shell is gone, so that it is necessary to coordinate shedding with other body systems and to behaviorally respond during the molting interval. Ecdysis or molting is coordinated by hormones. Aquatic crustaceans will reabsorb vital calcium salts before the current exoskeleton is cast off. An arthropod will molt from its current shell and then hide while the new shell hardens over a body that is"puffed up" with air or water. In summary, the exoskeleton is a versatile support for an arthropod, even as it requires modifications in movement and growth patterns.
(Sources: Week 10 lecture notes and handouts; Hickman et al., 7th ed., Ch. 23, pp. 491, 492, 500, 501, recitation video:"Life on Earth: The Segmented Invertebrates")
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Please send comments and questions to: cronewil@hvcc.edu
Copyright 1999 by Wilson Crone
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This page updated on November 12, 1999