242 



THE LAMELLIBRANCHIA 



however, the male and female portions of the gonad are not actually 

 contiguous, but are united by a short canal (Fig. 218, t, w\ so that the 

 spermatozoa have to pass through the ovarian cavity before they are 



Lyonsia nm-^egica, left-side view after removal of left pallial lobe and left gill, o, anus ; a.a, 

 anterior adductor ; a.s, anal siphon ; l.o, bysaal orifice ; hr.n, branchial nerve; br.s, branchial 

 siphon; /.o, female orifice; g, gill (direct inte..ial lamina); g', reflected internal lamina of 

 the right gill ; g", outer plate of the gill ; /.]>, labial palp.; m.o, male orifice ; o, ovary ; p.a, 

 posterior adductor ; p.o, pedal orifice of the mantle ; t, testis ; -e.g, visceral ganglion. 



eliminated. Finally, in the fourth mode, an ovary and a testis com- 

 pletely separated from one another exist on either side of the body, 

 each having its own proper duct anu external orifice. This is the 

 case in all the genera forming the order Anatinacea and in P&romya 

 among the Septibranchia. In these forms the ovary is dorsal and 



posterior, the testis more anterior 

 and ventral (Fig. 219, t.o). The 

 male and female genital orifices of 

 the same side are contiguous ; they 

 open on a common papilla in the 

 Anatinacea, but the female aper- 

 ture is outside the visceral commis- 

 sure, and therefore in the normal 

 and original position of the Lamelli- 

 branch genital orifice, whereas the 

 male aperture is within the visceral 

 commissure. In Poromya the male 

 and female ducts of each side open 

 into a common orifice, external to 

 the visceral commissure. In all 

 these hermaphrodites the male 

 products are the first to ripen. 



Pteudokellya cardiformis, section of a part Accidental CaSCS of hermaphro- 

 of the ovary, c^follicular cells ; ^, ovular difcism haye been met wifch j n 

 loilicie , oi, ovarian egg. t 



dioecious Lamelhbranchs (Mytilus, 



Unionidae), and a unisexual individual of the normally hermaphro- 

 dite species Pecten gkiber has also been described. 



Fio. 220. 



