80 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 Hmarginula, 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, Puncturella, and 
Haliotis. An analogous slit is also present in Stliquaria 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 
Fic. 63. 
Limacina antarctica, removed from its 
Fie. 62. shell, dorsal aspect. au, auricle; gl.pa, 
Schismope lacuniformis, seen pallial gland; go.se, seminal groove ; na, 
from the umbilicus. a, aperture right fin; pa, mantle; 7, kidney; te.d, right 
of the shell; 7, foramen; w, tentacle; te.g, left tentacle; ve, ventricle 
umbilicus. (After Watson.) 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 
