METAZOA MOLLUSCA. 217 



hole, while the coil portion of the shell becomes reduced 

 in size and is carried towards the posterior end (fig. 9). 

 This is seen on the edge of fig. 10, which is a ventral 

 view showing the foot, eyes, radula, and mantle. With 

 the growth of the shell the hole approaches the summit 

 (fig. n), until finally it occupies a nearly central position 

 with the vestige of the coiled shell behind it (fig. 12). 

 Thus it is seen that the symmetry of the adult (fig. 12 ; 

 see also No. 440) is not primitive but is secondarily 

 acquired. 



A fold of the mantle lies in the slit, and later occupies 

 the hole. It is thought to have an excretory function, 

 and if so, it serves as an anal siphon. Fissurella has the 

 original gills fully developed, one on each side of the 

 neck. The nephridia are also present. The heart has 

 two auricles and a ventricle. 



Some specimens of the Stomatellidae are limpet-like, 

 while others (No. ^\\^Stomatella imbricata Lam.), have a 

 spiral shell without perforations and with a large aperture. 



The young Haliotis has an imperforate shell, but in 

 the course of development a slit occurs in the margin 

 of the aperture through which the siphon is extended. 

 With the growth of the shell this slit is not continuous 

 but a series of holes is formed (see Nos. 442, 443) some 

 of which, according to Cooke, admit water to the gills, 

 while others are anal in function. With the growth of 

 the animal the first formed holes tend to become filled 

 with a limy deposit. The primitive gills and excretory 

 organs or nephridia are in pairs although the beginning 

 of the spiral has brought the anus from the posterior end 

 of the body towards the anterior median line. 



The specimen of Crucibulum (No. 444, C. striatum 

 Say), exhibits the fleshy body in the shell with its mantle 

 and muscular foot. The anterior part of the body has 

 become differentiated to form a head (see No. 444) which 

 bears sense organs. This specialization of the Gastro- 

 pods in general shows a marked advance over all the 

 classes of animals so far described. 



