VII 



MOLLUSGA—THE PALLIAL COMPLEX 



79 



2. In front of tlie anus, between it and the otenidium, is the nephridial aperture. 

 Following these there may be — 



3. A hypobranchial gland. 



4. The otenidium. ■ ^- - 



5. At the base of the otenidium or on its axis, 

 the osphradium. 



Were this complex of organs to be shifted along yKt\ . 8 



the edge of the body, we should have the arrange- 

 ment found in the MonotocanHa. among the Frosu- 



braiichia. The correspondence is, however, appar- . I Y^'j-V 9 



ently marred by the position of — "1 ( / P in 



6. The genital aperture, which in the Opistho- 

 branohia lies farthest forward of all the pallial 

 organs. -i^ 



In all other Opisthobranchia (after excluding the 1 --'^ H 



Tectibranchia) the pallial complex is broken up ^ i _, ., 



when the mantle and the true otenidium disappear. 5 ]_f A \ 



The only e.xoeption to this is found in the PJiyl- 6 -'" 



lidiidce, where, apart from the gills, a similar 1 / X.V°W" "» 



arrangement to that in the Tectibirmchia occurs. / Y\ ^5 



The single or paired genital aperture always lies , 



asymmetrically on the right side in front of the / 1 'O 



anus, which is sometimes found asymmetrically on 



the right side, and sometimes has a median dorsal 



position between the middle and the posterior end I ..s' 1/ A 



of the body. The renal aperture lies between the \ ^^^'%l(f>3^y- ffi 



anus and the genital aperture, sometimes close to 

 the latter. 



In the Pteruixjdu gymnosomata. (Fig. 76) the 

 shell and mantle are wanting. The otenidium, I ■i ? 1 1 »? 



when retained, as in the Z)f.j;ioft;ff)ic/ifa and P/iciHio- ' ^^ "* 



derma, lies somewhat far back on the right side of 

 the body, far behind the anus. On the disappear- ^ 



ance of the mantle, it evidently shifted back from ^^^ V6.-Pneumoderma, from the 

 its original position between the anus and the genital right side, diagrammatic (after Pel- 

 aperture, while the osphradium, which is generally Senear). 1, Right process bearing 

 found close to the otenidium, has, as far as has yet "^^°°^^ (Hakensack) evaginated ; 2, 

 1 , 1 J. • 1 -J. • • 1 -J.- proboscis ; 3, right buccal tentacle ; 



been observed, retained its original position. ■, -i- « *? ■ , i 1 , * 



' SI' 4^ position of the right nuchal ten- 



The anus lies anteriorly behind the right fin ; tacle ; 6, right flu (parapodium) ; 6, 



the nephridial aperture lies close by, either distinct seminal furrow ; 7, genital aperture ; 



or united with the anus at the base of a common S, position of the jaw ; 9, ventral pro- 



1 ,j . T j'^-i-fj-i-i-i- boscidal papilla : 10, right buccal ap- 



cloacal depression. Immediately m front 01 this , 1- , ,,, ; 



^ -^ pendage bearing suckers ; 11, head ; 



lies the osphradium, then follows, considerably 12, aperture of penis; 13, right anterior 



farther forward on the neck, to the right behind pedal lobe ; 14, anus ; 15, posterior 



the base of the right fin, the genital aperture, from Pe^al lobe; 16, otenidium; 17, pos- 



which, as in many TedihrancUa, a ciliated furrow *"'"'■ f Pf™ «"' ^ '\ ".' «■ V' <""^^^l> 



' -^ p /. 1 1 T ventral, anterior, posterior, 



runs forward along the surface 01 the body to tlie 



aperture of the penis, which lies to the right in front of the foot. 



All Thecosomata have a mantle and a mantle cavity, and often a shell as well ; 

 in the CymhuUidce, the latter is replaced by a cartilaginous pseudoconch, a, sub- 

 cutaneous formation of the mantle. 



Among the Thecosoinata, the Limacinidce indicate the primitive arrangement ; 

 they possess a dorsal or anterior mantle cavity, a coiled shell, and an operculum. 



