136 



MOLLUSCA 



sac opens by a pore into each nephridium instead of 

 directly to the surface. A single pair of ctenidia (gill- 

 plumes) is present instead of the two pairs in Nautilus. 

 The existence of two pairs of ctenidia and of two pairs 

 of nephridia in Nautilus, placed one behind the other, is 

 highly remarkable. The interest of this arrangement is in 

 relation to the general morphology of the Mollusca, for 

 it is impossible to view this repetition of organs in a linear 

 series as anything else than an instance of metameric seg- 

 mentation, comparable to the segmentation of the ringed 

 worms and Arthropods. The only other example which 

 we have of this metamerism in the Mollusca is presented 

 by the Chitons. There we find not two pairs of ctenidia 

 merely, but sixteen pairs (in some species more) accom- 



v.br 



Fio. 103. View of the postero-ventral surface of a male Sepia, obtained by 

 cutting longitudinally the firm mantle-skirt and drawing the divided halves 

 apart. This figure is strictly comparable with fig. 101. C, the head ; J, the 

 mid-foot or siphon, which has been cut open so as to display the valve i ; R, 

 the glandular tissue of the left nephridium or renal-sac, which has been cut 

 open (see fig. 108) ; P, P, the lateral fins of the mantle-skirt ; Br, the single 

 pair of branchije (ctenidia) ; a, the anus, immediately below it is the open- 

 ing of the ink-bag ; c, cartilaginous socket in the siphon to receive d, the 

 cartilaginous knob of the mantle-skirt, the two constituting the "pallial 

 hinge apparatus " characteristic of Decapoda, not found in Octopoda ; g, the 

 azygos genital papilla and aperture ; 'i, valve of the siphon (possibly the rudi- 

 mentary hind-foot) ; m, muscular band connected with the fore-foot and 

 mid-foot (siphon) and identical with the muscular mass k in fig. 91 ; r, renal 

 papillae, carrying the apertures of the nephridia; v.br, branchial efferent 

 blood-vessel ; v .br f , bulbous enlargements of the branchial blood-vessels (see 

 figs. 104, 108) ; , ink-bag. (From Gegenbaur.) 



panied by a similar metamerism of the dorsal integument, 

 which carries eight shells. In Chiton the nephridia are 

 not affected by the metamerism as they are in Nautilus. 

 It is impossible on the present occasion to discuss in the 

 way which their importance demands the significance of 

 these two instances among Mollusca of incomplete or partial 

 metamerism ; but it would be wrong to pass them by with- 

 out insisting upon the great importance which the occur- 

 rence of these isolated instances of metameric segmentation 

 in a group of otherwise unsegmented organisms possesses, 

 and the light which they may be made to throw upon the 

 nature of metameric segmentation in general. 



The foot and head of Nautilus are in the adult inex- 

 tricably grown together, the eye being the only part belong- 

 ing primarily to the head which projects from the all- 

 embracing foot. The fore-foot or front portion of the foot 



in Nautilus has the form of a number of lobes carrying 

 tentacles and completely surrounding the mouth (figs. 88, 

 89, 91). The mid-foot is a broad median muscular process 

 which exhibits in the most interesting manner a curling in 

 of its margins so as to form an incomplete siphon (fig. 

 101), a condition which is completed and rendered per- 

 manent in the tubular funnel, which is the form presented 

 by the corresponding part of Dibranchiata (fig. 96). The 

 hind-foot possibly is represented by the valvular fold on the 

 surface of the siphon-like mid-foot. In the Pteropoda the 

 wing-like swimming lobes (epipodia or pteropodia) corre- 

 spond to the two halves of the siphon, and are much the 

 largest element of the foot. The fore-foot surrounding 

 the head is often quite small, but in Clione and Pneumo- 

 dermon carries lobes and suckers. A hind-foot is in Ptero- 

 poda often distinctly present ; it is open to doubt as to 

 whether the corresponding region of the foot in Siphono- 

 poda is developed at all. 



The lobes of the fore-foot of Nautilus and of the other 

 Siphonopoda require further description. It has been 

 doubted whether these lobes were rightly referred (by 

 Huxley) to the fore-foot, and it has been maintained by some 

 zoologists (Grenadier, Jhering) that they are truly processes 

 of the head. It appears to the present writer to be im- 

 possible to doubt that the lobes in question are the fore- 

 portion of the foot when their development is examined 

 (see fig. 121, and especially fig. 72**), further, when the fact 

 is considered that they are innervated by the pedal ganglion, 

 and, lastly, when the comparison of such a Siphonopod as 

 Sepia is made with such a Pteropod as Pneumodermon in its 

 larval (fig. 84) as well as in its adult condition (fig. 85). The 



FIG. 104. Circulatory and excretory organs of Sepia (from Gegenbaur, after 

 John Hunter), br, branchiae (ctenidia) ; c, ventricle of the heart ; a, anterior 

 artery (aorta) ; a', posterior artery ; v, the right and left auricles (enlarge- 

 ments of the efferent branchial veins) ; v', efferent branchial vein on the free 

 face of the gill-plume ; v.c, vena cava ; vi, vc', advehent branchial vessels 

 (branches of the vena cava, see fig. 108) ; vc", abdominal veins ; x, branchial 

 hearts and appendages ; re, e, glandular substance of the nephridia developed 

 on the wall of the great veins on their way to the gills. The arrows indicate 

 the direction of the blood-current. 



larval Pneumodermon shows clearly that the sucker-bearing 

 processes of that Mollusc are originally far removed from 

 the head and close in position to the pteropodial lobes of 

 the foot. By differential growth they gradually embrace 

 and obliterate the head, as do the similar sucker-bearing 

 processes of Sepia. In both cases the sucker-bearing pro- 

 cesses are "fore-foot." The fore-foot of Nautilus completely 

 surrounds the buccal cone (fig. 88, e), so as to present an 

 appearance with its expanded tentacles similar to that of the 

 disc of a sea-anemone (Actinia). No figure has hitherto 

 been published exhibiting this circum-oral disc with its 

 tentacles in natural position as when the animal is alive and 

 swimming, the small figure of Valenciennes being deficient 

 in detail. All the published figures represent the actual 

 appearance of the contracted spirit-specimens. Mr A. G. 



