. Animal physiology. Physiology, Comparative; Physiology, Comparative. 64 GENERAL STRUCTURE OF MOLLUSCA. parts of the body, and are connected with the preceding by nervous cords. The former correspond to those contained in the head of insects; but of the latter, one only is connected with the foot or organ of motion, the remainder having for their function to regulate the action of the gills, and to perform other movements connected with the operations of nutrition. In Fig. 15 is represented one of the simpler forms of this nervous system,—that of the Pecten or scal- lop-shell ; A A are the ga
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 64 GENERAL STRUCTURE OF MOLLUSCA. parts of the body, and are connected with the preceding by nervous cords. The former correspond to those contained in the head of insects; but of the latter, one only is connected with the foot or organ of motion, the remainder having for their function to regulate the action of the gills, and to perform other movements connected with the operations of nutrition. In Fig. 15 is represented one of the simpler forms of this nervous system,—that of the Pecten or scal- lop-shell ; A A are the ganglia near the mouth, from which the organs of sense are supplied; B is the ganglia connected with the gills; and C is that from which power is given to the foot. The two first lie wide apart, but are connected by an arched band that passes over the gullet, e. The organs of sense among the Mollusca are but little more developed than those of mo- tion. They serve to direct the animal to its food, and to warn it of danger; but there seems an absence, in all but the highest species, of that ready and acute sensibility which is so remark- able in the preceding groups; and the variety of impressions which they can receive appears to be but small. In no instance has a special organ of smell been certainly discovered ; the organ of hearing is always imperfect, and frequently absent altogether; and the eyes are very often wanting; so that touch and taste (which is but a refined kind of touch) are the only senses left. —The blood of the Mollusca is white, as it is in the Articulata; but the apparatus by which it is circulated through the body is much more powerful and complete. 79. The fourth and last subdivision, that of Radiata, includes those animals which have the parts of the body arranged in a circular manner around a common centre, so as to present a radiated or rayed aspect. This arrangement is well seen in the common Star-fish, which has five such rays, all having a pre-. F
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