MOLLUSCS. 



287 



Glass H. — CEPHALOPHOEA. 



The remainder of the Mollusca differ greatly from the group which we have just 

 left, and the fact that nearly all of them possess a lingual ribbon (an organ to be 

 described further on), while there is a head distinct from the rest of the body, has 

 caused them to be included in one large group, variously termed Glossophora, Odon- 

 tophora, and Cephalophora. In the present work, however, the term Cephalopliora 

 is restricted to indicate the forms between the squids and cuttle-fishes (Cephalopoda) 

 and the Acephala. 



The anterior end of the body is more or less distinctly marked off as a head, and 

 this differentiation is the more marked in many forms from the fact that it usually 

 bears tentacles and eyes, and thus is seen to be the locality of the senses, increasing 

 its claim to the term head. The body possesses a bilateral symmetry, but, owing to 

 the fact that most of the forms live in spiral shells, this resemblance between the two 

 sides is somewhat obscured. The body is enveloped (at least in the young) in a 

 mantle comparable to that of the Acephals, which in most forms secretes the shell, 

 which is usually calcareous, but not infrequently, as in our common snails, is more or 

 less horny. As these shells are very important from a systematic point of view, and, 

 indeed, are the only portions usually preserved, they demand far more attention than 

 they otherwise would. 



The shells of the Cephalophora are always, except in the chitons, univalvular; that 

 is, composed of a single piece, which, though presenting the most various forms, can in 

 reality be reduced to a simple type. This type is a cone. The cone may be broad 

 and low, as in the limpets, or it may be greatly drawn out and very slender, as in the 

 tooth-shells. It may be coiled in a nearly flat spiral, or it may be curled in a conical 

 spiral, the form found in most of the shells of the group. 

 In some few forms the shell is internal, being enveloped 

 in a fold of the mantle, while in a large number of these 

 animals no shell is present. In the chitons the shell con- 

 sists of a number of pieces, never more than eight, 

 arranged in a linear series on the back. 



In systematic works each part of the shell has its 

 name. The upper spiral portion is known as the spire, 

 and the curved portions of which it is composed are 

 known as the whorls, the last and largest being the 

 body whorl. The whorls are separated from each other 

 by the sutures. The opening is known as the mouth 

 or aperture, the outer edge of which is the lip, the inner 

 the columella. Sometimes the lip is prolonged into one 

 or two grooves or canals which are always approxi- 

 mately parallel with the axis of the shell. The one pjg 317 

 nearest the spire is the posterior, the other the anterior 

 canal. Frequently an opening is left in the axis of the 

 shell, which is known as the umbilicus. 



Returning to the animal itself, the next thing we have to notice is the foot, which 

 is usually large and muscular, and is used as an organ of locomotion. It may bear on 

 either side a lateral appendage (epipodium), while frequently on the dorsal surface of 



Parts of a gasteropod shell; 

 a, anterior canal ; 6, body whorl; o, 

 columella; (, outer lip; m, mouth or 

 aperture; p, posterior canal; s, su- 

 tures; spf spire; u, umbilicus. 



