Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/701

Rh CEPHALOPODA.] MOLLUSCA 673 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 ; /, 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 branchise (ctenidia) ; a, the anus, immediately below it is the open ing of the ink-bag ; e, cartilaginous socket in the siphon to receive c, the cartilaginous knob of the mantle-skirt, the two constituting the &quot;pallial hinge apparatus &quot; 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) ; ro, muscular band connected with the fore-foot and mid-foot (siphon) and identical with the muscular mass k in fig. 91 ; r, renal papilla?, carrying the apertures of the nephridia; I .br, branchial efferent blood-vessel ; v.br 1, bulbous enlargements of the branchial blood-vessels (see figs. 104, 108); t, 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. 7 2**), 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 Fio. 104. Circulatory and excretory organs of Sepia (from Gegenbaur, after John Hunter). 6r, 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&quot;, 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 &quot;fore-foot.&quot; 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. XVI. 85