VII 



MOLL USCA OUTER ORGAN IS A TION 



31 



CVIA 



I 



Avvt 



T 



ue- 



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 opereulum, which, when the animal withdraws its head and 



foot, closes the aperture 



of the shell. The mantle 



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 \ 



' . , . . 'Utd 



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 tAvo 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.ii.-DiagramofaProsobranchiateMonotooardian. The 



nephndial aperture, and outer form, sliell, mantle, pallial complex, heart and perirardi , 



Osphradilim to the left aervous system and operculum, are depicted. Lettering mostly 



n , i -I as in Fig. 3'.i. In addition : /, foot ; si, siphon ; tup, xtiti. supr.-i- 



and sub -intestinal connectives ; op, operculum ; ^.auditory 



tailied, while the hind- organ: y, penis; sr, seminal groove; wh, mantle cavity; hy, 



"llt with the anUS moves n yi"-' w ' am ' llial gland ; ,5, male genital aperture ; /. rectum ; au, 



f . i, -I "" r , 1 -ye : ', tentacle. 



to the right side ot 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 original!}' 

 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. 



