VII 



M0LLU8GA— OUTER ORGANISATION 



31 



developed foot has a flat creeping sole. On the dorsal side of the 

 posterior portion, of the foot, the metapodium, there is a calcareous 

 plate, the operculum, which, when the animal withdraws its head and 

 foot, closes the aperture 



of the shell. The mantle 'v^ '^'^t 



fold hangs down from 

 the anterior side of the 

 visceral dome, and covers 

 the spacious branchial or 

 mantle cavity, in which 

 lie certain organs of 

 special morphological 

 importance. These, 



which may be called the 

 mantle or pallial organs, 

 are, in such forms as 

 may be considered primi- 

 tive, (1) the anus, which 

 lies, not posteriorly, but 

 on the anterior side 

 of the visceral dome, 

 shifted forwards to- 

 wards the mouth ; (2) 

 the two apertures of the 

 paired nephridia, one on 

 each side of the anus; (3) 

 the two gills, one to the 

 left and one to the right ; 

 (4) the two osphradia 

 near thebases of thegills. 

 In most Prosobranchia, 

 however, the organs just 

 mentioned as paired are 



unpaired; only the gill, fio. 4l.-DiagramofaProsobraiicliiateMonotocardlan.iTlie 

 nephridial aperture, and outerform, shell, mantle, pallial complex, heart and pericardium, 

 0=;r>hradiuni to the left nervous system and operculum, are depicted. Lettering mostly 



of the anus being re- 

 tained, while the hind- 

 gut with the anus moves 

 to the right side of the 

 mantle cavity. The single genital aperture lies on the right side, in 

 the head, or on the floor of the mantle cavity. (In the Prosobranchia 

 the sexes are separate.) The abortion of one of each of these originally 

 paired organs, gills, nephridia, and osphradia, produces a very striking 

 asymmetry of the whole body. The name Prosobranchia indicates the 

 fact that the gills lie in front of the heart. 



as in Fig, 39. In addition : /, foot ; si, siphon ; sup, suh, supra- 

 and sub - intestinal connectives; op, operculum; ot, auditory 

 organ ; p, penis ; sr, seminal groove ; mil, mantle cavity ; hy, 

 hypobranchial gland ; (5 , male genital aperture ; r, rectum ; au, 

 eye ; t, tentacle. 



