STRUCTURE OF THE MOLLUSCA. 191 



The heart, fig. 8, j, is usually composed of an auricle and a 

 ventricle, and, as in the other Mollusca, receives the arterial 

 blood coming from the organs of respiration. The mouth is 

 surrounded by contractile lips, and sometimes armed with 

 horny teeth fixed to the palate, or with a fleshy proboscis, 

 fig. 4, e. The stomach, liver, and other viscera, are lodged 

 beneath the mantle, and generally contained within the 

 shell. The intestine, fig. 8, m, almost always ends on the 

 right side of the body. 



In this class, the organs of sense are less developed than 

 in the Cephalopoda. The tentacula are organs of touch, 

 and perhaps of smell also. They have no special organ of 

 hearing. Their eyes, sometimes placed on the head, some- 

 times at the base of the tentacula, on their sides, or at 

 their tips, are always very small and simple, or sometimes 

 wanting. 



Their food is various ; some are terrestrial, others live in 

 fresh water, but the greater number in the sea. They form 

 the most numerous class of the Mollusca, and species of them 

 occur in all countries. 



The Gasteropodous Mollusca may be arranged into several 

 orders, respecting which there is much diversity of opinion, 

 and, as in every department of zoology, a great difference of 

 nomenclature. The orders proposed by Cuvier, and adopted 

 by many, are: — Pulmonata, Pectinibranchiata, Tuhuli- 

 hranchiata, Scutibranchiata, Tectibranchiata, Cyclobrancfd- 

 ata, Infer obranchiata, Nudibranchlata, Heteropoda, and 

 Cirrobranch ia ta . 



ORDER I.-OASTEROPODA PULMOMTA. 



The Gasteropoda which respire air differ from the rest in 

 having, in place of branchiae, a cavity on the back, or a kind 

 of lung, formed by a membranous bag, furnished internally 

 with a complex network formed by the pulmonary vessels, 

 and opening externally by a hole in the edge of the mantle, 



