OR OSSEOUS SYSTEM. 4J> 



(g,) at the base of which are the auricle and the ventricle (h.) 

 On the right side, within the mantle, is seen the anal orifice 

 (,) and the male organ (/.) The head (e?,)- provided with 

 two fleshy tentacula, with little black eyes at their base, is 

 extended from a short muscular neck, and projects from 

 the mouth a long powerful proboscis, furnished at its ex- 

 tremity with sharp recurved conical horny teeth (e.) The 

 closed and tapering part of the shell is occupied by the 

 turns of the liver and testicle in the male, or the liver 

 and ovary of the female, which organs accompany each 

 other to the apex of the spine. When the animal creeps 

 forwards, covered with its shell, it extends its foot, its head 

 and its neck from the aperture in such a mariner that 

 the upper lip of the shell lies over the free edge of the 

 mantle above the head, and the foot extends over the 

 lower lip, or the columella. The columella, or pillar, is the 

 axis of revolution, and is sometimes perforated through 

 its interior with a cavity called the umbilicus, which has 

 no communication with the cavity of the cone contain- 

 ing the animal, but tends to lighten massive shells, 

 where the wide revolutions are at a distance from each 

 other. 



As the calcareous layers of the shell are chiefly secreted 

 and formed by the anterior glandular portion of the 

 mantle, which is a part most liable to vary in its form 

 according to the age and the season, the chief differences 

 in gasteropodous shells are those produced by the diver- 

 sified forms of the upper lip. In the young animals 

 the upper lip of the shell is generally quite even and 

 smooth, and corresponds with the simple condition of 

 the margin of the mantle, but at maturity this upper 

 secreting portion of the mantle often assumes a highly deve- 

 loped, folded, or fimbriated edge, and the upper lip of 

 the shell takes a similar form. In the young strombus 

 gigas, the upper lip is quite even and uniform with the 

 ordinary turns of the cone, but at maturity it expands, 

 thickens, folds backwards, and becomes eifuse. The shell 

 of the pterocera scorpio, in its young state, (Fig. 23. 3,) 

 has the ordinary simple thin, incurvated margin of other 

 growing shells ; the canal, (Fig. 23. 3. &,) appears short 

 and truncated, and the apex of the spire (Fig. 23. 3. ,) pro- 



PART I. E 



