48 ORGANS OF SUPPORT, 



verse parallel imbricated pieces in the chiton. It is however 

 most generally in form of a hollow cone, wide and open at 

 the base, closed at the apex, and more or less convoluted 

 or spiral. This cone has been considered as analogous to 

 the two valves of the conchifera united together. It is 

 short, wide, and nearly straight in the patella, more length- 

 ened, narrow, and slightly bent in the dentalium, a little 

 twisted in the crepidula, the haliotis, and the capulus, and 

 it revolves round its apex in a single plane in the planorbis 

 and some of the helices, forming a flat disk like an ammonite. 

 In most of the gasteropodous shells, the plane of revolution is 

 constantly changing during growth, so as to cause the cone 

 to turn to the right side, and to revolve in a spiral manner 

 round an axis or pillar called the columella of the shell. This 

 spiral twist appears to be the result of the descent of the 

 foot over the columella, and the influence of the great 

 centres of the circulating and respiratory systems on the 

 left side of the body ; so that the apex of the spire is on the 

 right side of the animal, and the canal of the shell for 

 respiration is on its left side. In a few reverse shells, where 

 the spire lies to the left side of the aperture, the circulating 

 and respiratory organs are found transposed to the right side 

 of the animal. The general disposition of the soft parts in 

 the cavity of a spiral unilocular univalve shell will be seen 

 by this diagram of the common buccinum undatwn, Fig. 22, 

 where the animal is represented as creeping upon its ex- 

 panded foot. The upper and back part of the foot supports 

 the operculum (m,) which 



closes up the aperture of FIG - 2 ~- 



the shell when the ani- 

 mal is retracted. The 

 adductor muscle (b,) is at- 

 tached to the columella, 

 and is the only part of 

 the body adhering to the 

 shell. The mantle (i, g, , 

 k y e, /,) is open in front, 

 lines the aperture of the shell, secretes the calcareous layers, 

 and covers the respiratory organs (g.) The muscular funnel 

 (i,) extends through the canal on the left side of the aper- 

 ture of the shell, and leads the currents to the two branchiae 



