304 INVERTEBRATE MORPHOLOGY. 



municating with the exterior freely. In some forms the 

 mantle is slit from its margin upwards and backwards, a 

 corresponding slit occurring in the shell (Ji^marginida) . In 

 Haliotis and Pleurotomaria the slit in the shell becomes 

 closed at regular intervals, producing a row of round perfora- 

 tions, beneath which lies the mantle-slit, and through which 

 water finds a ready exit from the mantle-cavity, and, iu Fis- 

 surdla, in which at an early stage the margin of the shell 

 possesses a slit, by the subsequent growth and obliteration of 

 the spiral coiling the slit becomes converted into an aperture 

 which lies almost at the apex of the conical shell and leads 

 into the mantle-cavity, functioning as a means of exit of the 

 water and. excrementa from that cavity. In the greater num- 

 ber of forms, however, such slits or apertures do not exist ; 

 but one finds frequently the margin of the mantle produced 

 at one point on the left side into a projecting narrow lobe 

 whose edges may be brought into opposition, thus producing 

 a tube or siphon through which water may pass into the 

 mantle-cavity. Where this siphon is well developed a dis- 

 tinct notch is found in the margin of the shell, through which 

 it may be protruded, or else the lips of the notch are pro- 

 longed so as to form a grooved process, the siphonal canal, in 

 which the siphon lies, being by these arrangements able to 

 function even when the mouth of the shell is closed by the 

 operculum. In many forms the mantle-folds are sufficiently 

 large to allow of their being reflected over the outer surface 

 of the shell when the body is fully protruded. 



The foot is as a rule adapted for creeping, but in many- 

 cases is differentiated into pro-, meso- and metapodium, the 

 last usually bearing a chitinous or more or less calcified 

 operculum. In certain forms belonging to a group of pelagic 

 forms, however, which were formerly associated together as a 

 distinct order, the Heteropoda (Fig. 138), the pro- and meso- 

 podium are modified into a keel-like structure and bear a 

 peculiar sucker. The epipodium is frequently developed in 

 the Prosobranchia, especially in the more primitive species — 

 most frequently, however, being reduced to tentacle- or lobe- 

 like processes arising from the sides of the foot. 



In the majority (Fig. 1 33) of forms there is but a single 



