. Animal physiology. Physiology, Comparative; Physiology, Comparative. paratively small. The general structure of the animals of this group has been already described (§. 76). Some of them form shells, whilst others are destitute of them. The shells are com- posed of a single piece, or are univalve, excepting in one tribe ; and they have usually more or less of a spiral forma- tion. The animals of this class all possess a distinct head; and this is generally furnished with eyes, as well as with tentacula. They have often a power- ful masticating apparatus, and are voracious in their V^jSflH ha
. Animal physiology. Physiology, Comparative; Physiology, Comparative. paratively small. The general structure of the animals of this group has been already described (§. 76). Some of them form shells, whilst others are destitute of them. The shells are com- posed of a single piece, or are univalve, excepting in one tribe ; and they have usually more or less of a spiral forma- tion. The animals of this class all possess a distinct head; and this is generally furnished with eyes, as well as with tentacula. They have often a power- ful masticating apparatus, and are voracious in their V^jSflH habits; some of them feed upon vegetable matter, others upon animals. 124. The Acephalous Molluscaare divided into of paludina. two groups,—those which form shells, and those which do not. The former are termed Conchipera, or shell-bearing animals ; and this class includes all the Mollusca that form a shell composed of two parts or valves fitted together (which shell is termed bivalve), as well as some others whose general structure is the same, but whose shell is formed in several pieces, or multivalve. The shell in these animals sometimes attains an enormous size ; thus the shell and animal of the Tridacne gigas, or giant clamp- shell of the East Indies, have been known to weigh as much as 5 or 6 cwt. The two valves of a bivalve shell are connected by a hinge, where they are united by a ligament, which, by its elasticity, holds them apart while it keeps them together. This is their usual con- dition when the ani- mal is alive ; and in this manner, the water which is required for their respiration, and also to convey their supply of food, has free access to the internal parts. But when the animal is alarmed, and desires to close the shell for its protection, it does so by means of a muscle which stretches across from one valve to the other, and which, by con-. — Please note that these images are extracted from scanned page images that may have been digitally enhan
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