MOLL USCA—0 UTEE ORGAN ISA TION 



31 



developed foot has a Hat 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 '^N^ '^'^c o X/i^y 



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 jjallial organs, 



are, in such forms as 

 ' . _ \ 



may be considered primi- 

 tive, (1) the anus, which *^"'i 

 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 nejihridia, 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 the bases of the gills. 

 In most Prosobranchia, 

 however, the organs just 

 mentioned as paired are 



unpaired; only the gill, Fio.4l.-DiagrainofaProsobranchiateMonotocardian.lThe 



nephridial aperture, and outer form, shell, mantle, palllal complex, lu'urtau.l pericardium, 



OSphradium to the left "^rvous system and operculum, are depicted. Lettering mostly 



r ,1 I • as in Fig. 39. In addition :/, foot ; si, siphon ; «Mp, sjf'j, supra- 



01^ tne anus Demg re- and sub -intestinal connectives; ojj, operculum ; o<, auditory 



tained, while the hind- organ; p, penis; sr, seminal groove; mft, mantle cavity; hy, 



<^Ut with the anus moves 'lypobranclnal gland ; i, male genital aperture ; r, rectum ; au, 



* , . eye ; t, tentacle. 



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 sills lie in front of the heart. 



