56 MOLLUSC A. 



Prosobranchiates in consequence, because, in almost all, 

 the branchiae are also situated in front of the heart, as 

 in those described above. Chiton is a notable excep- 

 tion to this law, the heart receiving the blood and dis- 

 tributing towards the front, as it is also in the shell, 

 which is divided across the back into numerous over- 

 lapping plates, giving the whole animal the aspect of a 

 worm on the back. The foot, however, underneath, 

 always enables one to tell that it is a Gasteropod. The 

 shells of the marine and fresh water and land shells 

 of this group are often conveniently distinguished as 

 Siphonated and Holostomated, meaning siphon lipped 

 and entire lipped. The former (Figs. 37, Columbella; 

 40, Rostellaria) have a fold of the mantle pushed out 

 into a tube, through which water is drawn into the gills ; 

 and a canal in the aperture of the shell, often very long, 

 and sometimes a mere notch on the left side, holds 

 this, and the latter (Figs. 35, Acmea; 36, Crepidula; 

 30, 32, Paludina) have not this notch. 



The Siphonated Molluscs are generally carnivorous, 

 and the Holostomated vegetable-feeders ; but there are 

 so many exceptions to this rule, that it cannot be safely 

 applied to any one species whose habits are unknown. 



Besides the Prosobranchiates, there are other forms 

 usually classified under the name of Opisthobranchiates, 

 because the heart, when it is present, receives the blood 

 from behind, and sends it out through the aorta in 

 front. These are all marine, and though the shell is 

 quite fully developed in some species, most of the 

 forms have it in a rudimentary condition, or absent. 

 The mantle, also, is frequently absent, and the bran- 

 chiae wave above the back, free and uncovered in these 



