By Allan B. Cobb

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They have rings on the outside of the body that cause them to appear segmented. Many species lack a tail, and the cloaca is at the end of the body. They vary in size, with the largest species attaining lengths of up to three feet. Caecilians are fossorial or aquatic, and eyes are smaller than in the other two groups. In certain species the eyes are buried beneath the skin or even beneath skull bones. Caecilians also possess a unique sense organ called the tentacle, which is found between the nostril and the eye.

Berkeley: University of California Press, 1983. , and Jennifer A. Hurley. The Rights of Animals. , 1999. Sherry, Clifford J. Animal Rights: A Reference Handbook. , 1994. Singer, Peter. Animal Liberation: A New Ethics for Our Treatment of Animals, 2nd ed. New York: Random House, 1990. Animal Testing In order to more completely understand biology, researchers sometimes conduct experiments on animals. Animal experimentation has a lengthy and productive history in biological research, especially in biomedicine.

Bristleworms have a well-developed head region, often with tentacles, and well-developed sense organs, including paired eyes, antennae, and sensory palps (projections). They are unusual among annelids because their reproductive organs are developed only during the breeding season; afterward, they wither away. The sexes are separate. Gametes (eggs and sperm) are shed into the water, and fertilization is external. Development is indirect, via a trochophore larval stage. The polychaetes are believed to be the most primitive of the annelid classes.

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