126 ANIMAL KINGDOM. 



On the lower margin of the animal, toward the front part, there is, in the Clam and 

 most other species, a tough portion which is called the foot : it is used, when large, for 

 locomotion, as in the fresh-water Clam; when small, it sometimes gives origin to the 

 byssus by which shells like the Mussel are attached. It is wanting, or nearly so, in the 

 Oyster. 



The mantle is sometimes free at the lower margin, as in the Oyster; sometimes the 

 edges of the two sides are united, making a cavity about the body, open at the ends; in 

 other cases, this cavity is prolonged into a tube or siphon, or into two tubes projecting 

 behind, one receiving water for the gills, and the other giving the water exit. The 

 shell is closed by one muscle in the Oyster, etc., by two in the Clam, etc. The species 

 with two muscles are called Dimyaries, — from the Greek for two muscles; and those 

 with one, Monomyaries, — from the Greek for one muscle. 



These different peculiarities of the animal are partly marked on the shell. In Figs. 

 153, 154, the two muscular impressions are seen at 1 and 2; the impression of the 

 margin of the mantle (pallial impression, as it is called) at pp; and, in Fig. 154, the 

 siphon is indicated b} r a deep sinus in the pallial impression at s. In 155, the shell of 

 an oyster, there is only one muscular impression 



2. ASCIDIANS. 



Ascidians have a leathery or membranous exterior, bag-like, with 

 two openings, one for the admission of water and food, the other for 

 the exit of excretions. The name is from the Greek olctkos, a leather 

 wine-bottle. Having no shell, they are not yet known among fossils. 

 Yet it is probable that they were among the earliest kinds of Mol- 

 lusks. 



3. Brachiate Mollusks. 



1. Brachiopods. — Brachiopods (Figs. 150, 151, and 218 to 246, 

 pp. 171-173) have a bivalve shell, and in this respect are like ordinary 

 bivalves. But the shell, instead of covering the right and left sides, 

 covers the dorsal and ventral sides, or its plane is at right angles to 

 that of a clam. Moreover, it is symmetrical in form, and equal, either 

 side of a vertical line a b, Fig. 150 (p. 119). The valves, moreover, 

 are almost always unequal ; the larger is the ventral, and the other the 

 dorsal. There is often an aperture at the beak (near b, Fig. 150), 

 which gives exit to a pedicel by means of which the animal is fixed to 

 some support. In Fig. 151, p. 119, representing a species of the 

 genus Lingula, the fleshy support is a long one implanted in the 

 sand by burrowing. 



These Brachiopods are also peculiar in other points of structure. 

 They have a pallium, but no independent branchial leaflets. They 

 have a pair of coiled fringed arms, which in some Brachiopods may 

 be extruded (Fig. 226), — whence the name Brachiopod, meaning 

 arm-like foot. For the support of these arms, there are often bony 

 processes in the interior of the shell, of diverse forms in different 

 genera (Figs. 218, 222, and 225.) These arms serve to keep up a cur- 

 rent of water over or through the brachial cavity of the animal. 



