. Animal physiology. Physiology, Comparative; Physiology, Comparative. STRUCTURE OP ARTICULATED ANIMALS. 59 body. In general, these pieces are very similar to each other; so that the whole body appears like the repetition of a number of similar parts, as we see in the Centipede (Fig. 9). The limbs are usually very numerous, where they exist at all; and they have a jointed covering, like that of the body. But in the lower tribes of this group, such as Leeches and Worms, the limbs or members are but slightly developed, or are altogether absent; and in the highest, which approach most nearly to t
. Animal physiology. Physiology, Comparative; Physiology, Comparative. STRUCTURE OP ARTICULATED ANIMALS. 59 body. In general, these pieces are very similar to each other; so that the whole body appears like the repetition of a number of similar parts, as we see in the Centipede (Fig. 9). The limbs are usually very numerous, where they exist at all; and they have a jointed covering, like that of the body. But in the lower tribes of this group, such as Leeches and Worms, the limbs or members are but slightly developed, or are altogether absent; and in the highest, which approach most nearly to the Vertebrata in their general organisation, the number of members is much reduced, —although it is never less than six. The hard matter of which the external skeleton is composed, undergoes little or no change when it is once fully formed; and, in order to accommodate it to the increasing size of the animal, this cover- ing is thrown off and renewed at intervals. 71. The nervous system consists of a series of small masses or ganglia^ which are arranged in a cord or chain along the central line of the body. There is usually a large ganglion in the head, bearing a resemblance (in its peculiar connection with the eyes) to a certain part of the brain of Vertebrata; and there is com- monly one for each segment or division of the body, from which the nerves pass to supply its muscles, as they do from the spinal cord of Vertebrata. The cord which connects these ganglia is double; and the ganglia themselves are composed of two halves which have little connection with each other. The chain thus formed, passes along the under side of the trunk of the animal (as seen at g, Fig. 11), not on what seems its back; and it is by the presence of this double chain of ganglia that an Articulated animal maybe distinguished, even when, in its general structure, it should seem to belong to the group of Mollusca (see §. 113). 72. The general arrangement of the organs in the Articulata. Fig. 10.—
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