CHAPTER X. 



Molluscs, — continued. 



The Gastropods, — Class Gastropoda. 



The Gastropods constitute by far the largest division of the Mollusca, and include 

 among their number forms which bear no external resemblance whatever to one 

 another. Some are free-swimming animals, living far from land, out on the open 

 seas ; others occur in shallow water, or between tide-marks ; while others are 

 dwellers on the land, or frequent rivers and lakes. Some have internal, others 

 external shells, whilst many have no testaceous structure of any description. 

 Snails, whelks, and periwinkles are typical forms of Gastropods, and the more 

 aberrant types are represented by the Nucleobranchs, Heteropods, and Pteropods. 

 The typical Gastropods are all crawlers, moving like a slug or snail by a continual 

 expansion and contraction of the muscular foot. Some breathe by means of gills, 

 others by a lung, while certain forms are provided with both modes of respiration. 

 They are generally furnished with a spiral shell when adult. They are mostly 

 unsymmetrical animals, lying spirally coiled within the shell ; this want of symmetry 

 being particularly manifested in the breathing-organ. In many marine forms there 

 is only a single gill, but in a few genera — Fissurella for example — the gills are 

 paired. There is always a more or less distinct head, bearing one or two pairs of 

 tentacles, and there are generally a pair of eyes situated at the base or end of the 

 tentacles; or raised upon short stalks. The mouth is usually provided with one or 

 more jaws, and the lingual ribbon, or radula, within the mouth, varies greatly 

 in its armature, and plays an important part in the various schemes of classification 

 which have been proposed. While enormously developed in some groups, such as 

 the limpets, in a few it is entirely wanting. It consists of a thin chitinous mem- 

 brane, the surface of which is beset with a multitude of so-called teeth, 

 symmetrically arranged in transverse or oblique series. The teeth are siliceous, 

 insoluble in acid, and capable of rasping away hard substances. With it the whelk 

 and other carnivorous forms bore through the shells of bivalves, and the limpet 

 eats away the calcareous nullipore. Not only is the form of the teeth extremely 

 variable, but their number varies enormously in different groups. In an Eolis, one 

 of the Nudibranchs, there are but sixteen teeth ; in a Doris, belonging to tin' same 

 group, there are as many as six thousand, whilst in a large species of Helix the 

 number has been estimated at nearly forty thousand. The shells of Gastropods 

 are usually spirally coiled as in the snail, but sometimes they are tubular or conical, 

 like that of the limpet. The forms of spiral shells are innumerable and very 

 unlike; some being globose, with simple rounded aperture, while others are narrow 

 and long, with prolonged spires, and the mouth produced into a long anterior beak. 



