CHAP. III. TRIBES OF THE GASTEROPODA. 57 



they can dilate or contract at pleasure : hence they 

 have no gills, " but merely a network of pulmonary 

 vessels, which spread over the parietes and the roof of 

 their respiratory cavity." The tentacula, in such as 

 inhabit fresh water, are generally only two ; but in the 

 land and amphibious groups, as HelidC, Ampullaria, Pla- 

 norbis, Sic, they are four. Their food, notwithstand- 

 ing all these variations, is always essentially vegetable. 

 Their mouth is uniformly the same, and their shell is 

 always spiral ; this latter circumstance, it should be 

 remembered, carries with it an important character in 

 the form of their body, which is of much consequence, 

 and separates them from the next tribe. The whole 

 of the land shells, as well as the marine families of 

 Neritinw, Tiirbidce, TrocMdce, &c., are comprised in 

 this division ; the Haliotidcp, or ear-shells, being the 

 connecting link to the next. 



(49.) The ScuTiBRANCHiA have the same system of 

 respiration, according to Cuvier, as his Pectinibranchia ; 

 but they differ not only in their mode of reproduction, 

 but most remarkably in the form of their body. They 

 are all attached to their shelly covering by an amazing- 

 strong muscle, which fills the centre, and gives them 

 such an adhesive power, that they affix themselves im- 

 moveably to other substances, from which they can 

 only be separated by the sudden insertion of some sharp 

 instrument. They are, in fact, affixed Testacea ; for 

 although the animals can move about, they cannot live 

 but upon a substance where they can instantaneously 

 fix themselves. In their internal structure, we find the 

 heart, according to Cuvier, " traversed by the rectum, 

 and receiving the blood from two auricles, as is the case 

 in the greater number of bivalves." * The form of 

 the shell is no less peculiar ; its general shape is that of 

 a loAv, broad-based pyramid, or, if viewed inside, of a 

 very wide but shallow funnel. The common limpet, 

 in short, appears to us the most typical of the whole ; 



* It is by this group, in fact, as will subsequently be shown, tliat we 

 consider the Gasteropoda and the Bithyra are united. 



