8o 



THE GASTROPODA 



The pallial cavity is largely open in the Streptoneura ; its orifice 

 is narrower in the Tectibranchs (Fig. 148, IV), and is much reduced 

 in the Pulmonates (Figs. 67 and 177), as the result of the almost 

 complete fusion of the mantle border with the neck. 



The mantle border projects somewhat beyond the shell that 

 covers it, and may be furnished with little tentacles, with glands 

 and pigment spots. The border is not continuous in the most 

 archaic forms, but presents in the median line, or at a neighbouring 

 point morphologically equivalent to the median line, a more or less 

 deep longitudinal slit, as may be seen in the Pleurotomariidae (Figs. 

 54, 127, and 128), in Emarginula, and Scutum. This slit corresponds 

 in position with the extremity of the rectum, and admits of a more 

 rapid expulsion of the excrements and the respiratory fluid. The 

 edges of the slit may fuse together at one or more points, leaving 

 one or more orifices in the mantle and in the shell on the dorsal 

 side of the pallial cavity (Fig. 62), as in Fissurella, Pundurella, and 

 Haliotis. An analogous slit is also present in Siliquaria and Pleuro- 

 toma. In the female Vermetus 

 there is a median slit in the 

 border of the mantle (Fig. 

 45), but no corresponding 

 fissure in the shell ; in this 



Fio. 62. 



Schismope lamniformis, seen 

 from the umbilicus, a, aperture 

 of the shell ; /, foramen ; u, 

 umbilicus. (After Watson.) 



FIG. 63. 



Limacina antarctica, removed from its 

 shell, dorsal aspect, aw, auricle ; gl.pa, 

 pallial gland ; go.se, seminal groove ; na, 

 right fin ; pa, mantle ; r, kidney ; te.d, right 

 tentacle ; te.y, left tentacle ; re, ventricle 

 of heart. 



case the mantle slit admits of the fixation of the eggs to the 

 internal wall of the shell, to which they remain attached up to the 

 time of hatching. 



At the left or anterior corner of the pallial aperture the mantle 

 edge is often produced into a tube with a ventral slit (Fig. 99, XV) ; 

 this tube or siphon serves to admit water into the pallial cavity. 

 A siphon exists only in specialised Streptoneura ; it is but slightly 

 developed in the Cerithiidae, is rather larger in the Strombidae 

 (Fig. 75, si), and attains its greatest dimensions in the Cassididae and 

 Doliidae and in all the Rachiglossa and Toxiglossa. In the Volu- 

 tidae the siphon is furnished with an internal appendage. Ampul- 

 laria also possesses a long siphon, which may serve either for 



