672 



ZOOLOGY 



SECT. 



shell. 



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 opei 

 mouth or peristo 1 ) 

 shell. Starting ; 

 apex along the 

 cavity of the spira 

 cone, in order to r< 

 mouth in an aduM 

 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 dcxtral one. The 

 axis (Fig. 572) is in the 

 shape of a twisted shelly 

 rod the columella con- 

 taining a narrow lumen ; it 

 is formed by the close union 

 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 lines 

 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 strife 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 

 position which the mouth of the shell has occupied 



FIG. 571. Shell of Triton nodiferus. 



Natural size. 



the 



