STRUCTURE OF GASTEROPODS. 



putting its head or foot out of its shell. In this latter case, the 

 shell has on its edge a notch or canal, destined to lodge the re- 

 spiratory tube ; this notch is near the ternnination of the columella, 

 and on the side opposite to that towards which the spire is di- 

 rected. Consequently this canal is to the left, in the commoQ 

 species, and to the right, in perverse shells. 



5. The heart (^^■. 21, c) is always aortic : it is generally conn- 

 posed of an auricle and a ventricle, and its position, as is the 

 case with the respiratory tube, is in relation to the direction of 

 the spire of the shell. This organ is situate on the same side 

 with the tube, that is, on the side opposite to that towards which 

 the shell turns, and the procreative organs (the form of which 

 varies much) are always placed on this latter side, and conse- 

 quently on the side opposite to the heart. 



6. The mouth is surrounded by contractile lips, and sometimes 

 armed with horny teeth which occupy the palate. In many other 

 animals of this class, the anterior part of the oesophagus is very 

 fleshy, and possesses the faculty of projecting itself externally, in 

 such a manner as to constitute a trunk. Sometimes the stomach 

 is also furnished with cartilaginous or bony pieces, proper for 

 dividing food; the intestine is folded on itself and lodged between 

 the lobes of the liver and ovary; and the anus {fig' 21, a) is 

 almost always situate on the right side of the body. 



7. In this class, the organs of the senses are less developed 

 than in the cephalopods ; the tentacles, which most gasteropods 

 bear on their front, varying in number from two to six, serve 

 chiefly for tact and perhaps for smell. No organ of hearing 

 has been found, and their eyes, which are sometimes wanting, 

 are very small, and of a very simple structure: they resemble 

 those of the nautilus, and are sometimes adherent to the head, 

 and sometimes borne at the base on the side, or at the end of the 

 tentacula. 



8. The class of gasteropods is divided into eight orders, the 

 principal characteristics of which are derived from the disposition 

 of the branchial (hrank-e-al) apparatus, as may be seen in the 

 following table ; 



5. What is the character of the heart in ga'steropods ? 



6. What are the peculiarities of the digestive organs? 



7. Where are the tenta'cula placed ? What is the number of tentacles in 

 ga'steropods ? Where are their eyes situated 7 



8. Into how many orders is the class of ga'steropods divided ? Upon what 

 are these divisions founded ? 



