CHAPTER X 



MOLLUSKS continued 



THE GASTROPODS Class Gastropoda 



THE Gastropods constitute by far the largest division of the Molluska, 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, while 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 sym- 

 metry 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 membrane, 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 

 the same group, there are as many as six thousand, while 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 tubu- 

 lar 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. The color and ornamentation of the surface are also as varied as 



