Class 3. GASTEROPODES. 337 



less distinct, according as it is more or less drawn in under the cloak. It is furnished 

 with tentacula of [comparatively] small size, and which do not encircle the mouth ; 

 their number varies from two to six, but they are sometimes wanting ; they are organs 

 of touch, and, at most, of smell also. The eyes are very small, sometimes placed upon 

 the head, sometimes at its base, either to a side or at the tips of the tentacula ; they 

 are sometimes also wanting. I'he position, the nature, and the structure of their 

 breathing organs vary, and afford characters whereby to divide them into several 

 families ; but they have never more than one aortic heart, — that is to say, one placed 

 between the pulmonary vein and the aorta. 



The position of the orifices of the organs of generation, and that of the anus, varies; 

 but they are almost always on the right side of the body. 



Several are entirely naked, others have only an interior shell, but the majority are 

 covered with one which contains the soft body, and shelters it. 



These shells are secreted in [or on] the cloak. Some of them consist of several 

 symmetrical pieces [or valves] ; some of a single symmetrical piece ; and others of a 

 non-symmetrical piece, and when this is very concave, or continues to grow for a long 

 time, an obliquely spiral shell is necessarily produced. In fact, that the shell may 

 represent an oblique cone, on which are placed successively other cones always wider 

 in one direction than in the others, it is necessary that the whole should turn to the 

 side which enlarges the least. 



That part upon which the cone is turned is named the columella [or pillar] , and it is 

 sometimes full, and sometimes hollow. In the latter case, its opening is called the 

 umbilicus. 



The whorls of the shell may remain nearly on the same plane, or they may tend 

 always towards the base of the columella. In this case, the preceding whorls rise above 

 the others as they are formed, and constitute what is called the spire, which is acute 

 in proportion to the rapidity with which the whorls descend, and to the measure of their 

 increase. The shells with an elongated spire are said to be turbinate. When, on the 

 contrary, the whorls remain depressed on the same level, and do not envelope each 

 other, the spire is flat, or even concave. These shells are called discoid. When the 

 upper part of each whorl envelopes or covers the preceding, the spire is concealed. 



The place in the shell whence the animal protrudes itself, is named the mouth, or 

 aperture. 



When the whorls remain nearly on the same plane, the animal, in creeping, has its 

 shell placed vertically, the columella transversely upon the hinder part of the back ; 

 and its head passes out under the margin of the mouth opposite to the columella. 

 When the spire is turbinate, the whorls turn obliquely to the right side in nearly all the 

 species, but in a small number to the left ; and the latter are named reversed, [or 

 sinistrorsaT] . 



It is to be observed that the heart is always on the side opposite to that to which the 

 spire is directed. It is, consequently, in general on the left, and only on the right in 

 the reversed kinds. The contrary is the rule with the organs of generation. 



The organs of respiration, which are always within the last whorl of the shell, receive 

 the circumfluent element under its margin, sometimes because the cloak is detached 

 from the body along the whole of this margin, and sometimes because it is perforated 

 tliere with a hole. The margin of the cloak, in many Gasteropods, is prolonged into 



