672 



ZOOLOGY 



SECT. 



The shell (Fig. 571) is a very hard and dense calcareous 

 structure, presenting no trace of division into the valves com- 

 posing the shell of the fresh-water Mussel, and lacking also 

 its bilateral symmetry. It is in the form of an elongated hollow 

 cone closely wound round a central axis. The apex of the cone 

 is the organic apex of the shell, corresponding to the umbo of 

 the fresh-water Mussel, and is the point from which the growth 

 of the shell has proceeded; the base is represented by the 



wide oblique opening the 

 mouth or peristome of the 

 shell. Starting from the 

 apex along the internal 

 cavity of the spirally wound 

 cone, in order to reach the 

 mouth in an adult shell, 

 we have to pass completely 

 round the central axis five 

 times i.e. the spiral con- 

 sists of five turns. In fol- 

 lowing the turns, the direc- 

 tion taken is to the right, 

 that is to say, the spiral 

 of the shell is a right- 

 handed or dc,rtral one. The 

 axis (Fig. 572) is in the 

 shape of a twisted shelly 

 rod the columclla - - con- 

 taining a narrow lumen ; it 

 is formed bv the close union 



t/ 



of the axial portions of the 

 wall of the spiral. The 

 windings of the spiral are 

 marked on the outer sur- 

 face of the shell by a nar- 

 row impressed spiral line or 

 suture, parallel with which 

 are numerous fine ridges 

 and depressions the lin<-x 

 of growth ; the increase in size of the shell takes place in 

 the direction of these lines, not at right angles to them as in 

 the shell of the fresh-water Mussel, and the lines that more 

 strictly correspond to the lines of growth of the latter are 

 excessively fine stria? which run transversely to the stronger 

 lines. At certain points, usually three in a full-grown shell, 

 the spiral is interrupted by a transversely directed edge which 

 appears to overlap the succeeding portion : this edge marks 

 the position which the mouth of the shell has occupied 



FIG. 571. Shell of Triton nodiferus. 



Natural size. 



