VII 



MOLLUSGATHE PALLIAL COMPLEX 



79 



SI- 



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

 Following these there may be 



3. A hypobranchial gland. 



4. The ctenidium. 



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

 the osphradium. 



Were this complex of organs to be shifted along 

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

 ment found in the Monotocardia among the Proso- 

 branchia. The correspondence is, however, appar- 

 ently marred by the position of 



6. The genital aperture, which in the Opistho- 

 branchia lies farthest forward of all the pallial 

 organs. 



In all other Opisthobranchia (after excluding the 

 Tcctibranchia) the pallial complex is broken up 

 when the mantle and the true ctenidium disappear. 

 The only exception to this is found in the Pliyl- 

 lidiidce, where, apart from the gills, a similar 

 arrangement to that in the Tcctibranchia occurs. 

 The single or paired genital aperture always lies 

 asymmetrically on the right side in front of the 

 anus, which is sometimes found asymmetrically on 

 the right side, and sometimes has a median dorsal 

 position between the middle and the posterior end 

 of the body. The renal aperture lies between the 

 anus and the genital aperture, sometimes close to 

 the latter. 



In the Pteropoda gymnosomata (Fig. 76) the 

 shell and mantle are wanting. The ctenidium, 

 when retained, as in the Dcxiobranchia and Piieuno- 

 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 

 its original position between the anus and the genital 

 aperture, while the osphradium, which is generally 

 found close to the ctenidium, has, as far as has yet 

 been observed, retained its original position. 



The anus lies anteriorly behind the right fin ; 

 the nephridial aperture lies close by, either distinct 

 or united with the anus at the base of a common 

 cloacal depression. Immediately in front of this 

 lies the osphradium, then follows, considerably 

 farther forward on the neck, to the right behind 

 the base of the right fin, the genital aperture, from 

 which, as in many Tcctibranchia^ a ciliated furrow 

 runs forward along the surface of the body to the 

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



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

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

 cutaneous formation of the mantle. 



Among the Thecosomata, the Limacinidce indicate the primitive arrangement ; 

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



17 



FIG. 76. Pneumoderma, from the 

 right side, diagrammatic (after Pel- 

 seneer). 1, Right process bearing 

 hooks (Hakensack) evaginated ; 2, 

 proboscis ; 3, right buccal tentacle ; 

 4, position of the right nuchal ten- 

 tacle ; 5, right fin (parapodium) ; 6, 

 seminal furrow ; 7, genital aperture ; 

 8, position of the jaw ; 9, ventral pro~ 

 boscidal papilla ; 10, right buccal ap- 

 pendage bearing suckers ; 11, head ; 

 12, aperture of penis ; 13, right anterior 

 pedal lobe ; 14, anus ; 15, posterior 

 pedal lobe ; 16, ctenidium ; 17, pos- 

 terior adaptive gill ; d, v, a, p, dorsal, 

 ventral, anterior, posterior. 



