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. 



875. 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 Moliusks 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-fish tribe. 

 It is in this group, that 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 through the water 

 with great velocity. 



III. GASTEROPODA, which form the most extensive group of 

 the whole Sub-Kingdom. These have a single foot, or muscular 

 disc for locomotion, formed by a thickening of the mantle on the 

 under surface of the body. Whilst the animals of the two pre- 

 ceding classes are entirely marine, there are species among these 

 which are adapted to live in fresh water, and even on land. 



We are probably to add to these the group of HETEROPODA, 

 which has been usually ranked as an Order of the preceding, but 

 which has been lately found to differ from them so much in 

 internal structure, as well as in external form, as to be entitled 



