MOLLUSKS. 157 



group excepted, are herbivorous, while those having a beak are as generally 

 carnivorous. 



3. Acephals, or Headless Mollusks. — There is but one group. 



Conchifers or ordinary Bivalves, also called Lamellibranchiates. — 

 These common species are well known as bivalves. Between the 

 mantle or pallium and the body of the animal lie the lamellar 

 branchiae, or gills, as is obvious in an oyster ; and hence the name 

 Lamellibranchiates. In a shell like fig. 163, the mouth of the animal 

 faces almost always (except in some species of Nucula and Solemya) 

 the margin a, or the side of the shorter slope ; and a is therefore the 

 anterior side, b the posterior ; and, placing the animal with the short 

 slope in front, one valve is the right, and the other the left. The 

 hinge is at the back of the Mollusk. 



On the lower margin of the animal, towards the front part, there is in the 

 Clam and many 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 ; some- 

 times 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 JDimya- 

 ries, — from the Creek 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. 163, 164, 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 p p; and in 

 fig. 161 the siphon is indicated by a deep sinus in the pallial impression at s. 

 In 165, the shell of an oyster, there is only one muscular impression. It is 

 observed also that in fig. 163 about one-third of the animal would be anterior 

 to a vertical line (m) let fall from the hinge ; whereas in the Oyster, Avicula, 

 Mytilus, and related species, the animal is almost wholly posterior to this line : 

 in other words, the Oyster is all venter, while the Clam is a higher type in the 

 order of Acephals. 



The remaining Mollusks are of a distinct type, namely: — 

 The Anthoid Mollusks, many having stems like flowers. 



1. Ascidians, or Tunicates. — These Mollusks are enclosed in a 

 leathery skin instead of a shell. They do not occur fossil. 



2. Bryozoans. — The Bryozoans, or moss-animals (so named from the 

 moss-like corals they often form), look like polyps, as represented 

 in figs. 162, 162 a. 162 is magnified about eight times. The corals 

 consist of minute cells, either in branched, reticulated, or incrust- 

 ing forms, and are common in the Silurian as well as later rocks 



