336 CLASSIFICATION OF MOLLUSC A. 



last case it may be so disposed as to admit the flow of water 

 introduced from without over its surface, as we shall see among 

 the Tunicated Mollusks ; or it may be adapted to expose the 

 blood to the influence of air, as is the case among the Terrestrial 

 Gasteropods (the Snail, Slug, and their allies), which are the 

 only Mollusca not aquatic. 



947. In the classification of the Mollusca, the system of 

 Lamarck will be generally followed, with some modifications 

 rendered necessary by the researches of others. The Sub- 

 Kingdom may be first divided, like that of Articulata, into two 

 principal sections ; the first including all those which have a 

 distinct head, or (in other words) which have the mouth situated 

 on a prominent part of the body, furnished with organs of special 

 sense ; the second comprehending those in which no such head 

 exists, the mouth not being capable of being projected beyond 

 the body, and no organs of special sense being present. The 

 former may be termed Cephalous, and the latter Acephalous 

 (or headless) Mollusks. The Cephalous Mollusks are usually 

 divided into the three following classes : 



I. CEPHALOPODA, which have feet or tentacula arranged in a 

 circular manner around the head, as in the Cuttle-Jlsh tribe. 

 In this group we find the nearest approach to the Vertebrata. 



II. PTEROPODA, a small but interesting class, characterised by 

 the possession of a pair of wing-like expansions of the mantle, 

 which serve as fins, and enable them to swim with great velocity. 



III. GASTEROPODA, the most extensive group of the Sub-King- 

 dom, have a single foot, or muscular disc for locomotion, formed 

 by a thickening of the mantle on the under surface of the body. 

 The animals of the two preceding classes are entirely marine ; there 

 are species among these which live in fresh water, and even on land. 



With the Gasteropoda we also place the aberrant group of 

 HETEROPODA, which has been regarded by many authors as 

 forming a distinct class. The animals composing it are especially 

 distinguished by the form of the foot ; which, instead of being a 

 horizontal disc, is compressed vertically, so as to form a fin, 

 which is turned upwards, instead of being situated below the body. 



948. In each of the foregoing Classes, we observe a consider- 

 able variation in regard to the relative size, and even the very 



