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These animals are not generally found where they can be readily observed in nature, but many valuable observations can be made on specimens confined in aquaria. If other animals are present in the aquarium notice the position of defense that is taken. In nature the animal probably spends much of its time under rocks with the anterior end of the body turned toward the opening. In this position both sense organs and weapons are in the proper position for attack or defense. Notice how the appendages are used. Are the sense organs moved frequently? Why are the eyes on stalks? What appendages are used in walking? Are all of these appendages used in just the same way? Does the animal move equally well in all directions?

Perhaps you can make the animal swim; if so, observe the method. Feed a specimen with portions of a clam or fish, and see how food is torn to pieces and transferred to the mouth, and determine, if possible, how the mouth appendages are used.

External Anatomy. As in Nereis, the body is segmented. The metameres of the head and thorax, however, are immovably fused to form a cephalo-thorax. This is covered dorsally by a single piece, the carapace.