204 



IXVEKTEBEATE ANIMALS. 



Fig. 143. — Scalaria 

 GrienlauiUcny a Ho- 

 lostoinatous Uni- 

 valve. 



be noticed. The shell is composed either of a single piece (univalve), 

 or of a number of plates placed one behind the other (multivalve). 



The univalve shell is to be looked upon as essentially a hollow 

 cone, the apex of which is placed a little on one side. In the 

 simplest forms, as in the Limpets, the conical 

 .shell is retained throughout life without any 

 alteration. In the great majority of cases, how- 

 ever, tlie cone is considerably elongated, so as to 

 form a tube, which may retain this shape (as in 

 the " Tooth-shell "), but which is usually coiled 

 up into a spiral. The " spiral univalve " may, in 

 fact, be regarded as the typical form of the shell 

 in the Gasteropoda (fig. 140, B). The coils of the 

 spiral are termed the " whorls," and are usually 

 more or less amalgamated on one side. In most 

 cases, too, the whorls aie wound obliquely round 

 a central axis or pillar, increasing gradually in 

 size to the mouth. The last whorl is the largest, 

 and is termed the " body-whorl." The mouth 

 of the shell in many forms is unbrokenly round 

 or " entire " (fig. 143), and it is found that most 

 of these forms subsist on vegetable food, as, 

 for instance, the common Periwinkles. In others, again (fig. 140, 

 B), the mouth of the shell is notched ov is produced into a canal, as 

 in the common Whelk, and it is found that these live upon animal 

 food, or are " carni\orous." There may be more than one of these 

 canals or tubes, but they do not necessarily indicate the nature of 

 the food, as their function is to protect the respiratory siphons. 



The Gasteropoda are divided into a good many groups, of which 

 the more important may be briefly noticed, the foregoing applying 



chiefly to the ordinary forms, which 

 therefore need no further description. 

 The remaining members of the water- 

 breathing Gasteropods are divided 

 into two sections, differing a good deal 

 from the typical forms of the class in 

 many respects. 



As examples of the first of these 

 may be taken the Sea-shigs and Sea- 

 lemons (Xudihranchiata), specimens of which may at any time be 

 found creeping about on sea-weeds, or attached to the under sur- 

 face of stones at low water. These slug-like animals (fig. 144) are 

 wholly destitute of a shell when fully grown, but possess an em- 

 bryonic shell when young. When there are any distinct respiratoi-y 



Fig. 144. — Nudibrancliiata. Doris 

 Johiistoni, one of the Sea-lemons. 



