MOLLUSCA 



137 



Bourne, B.Sc., of University College, has prepared from 

 actual specimens the drawings of this part in the male and 

 female Xautilus reproduced in fig. 88, and has restored the 

 parts to their natural form when expanded. The drawings 

 show very strikingly the difference between male and female. 

 In the female (lower figure), we observe in the centre of 

 the disc the buccal cone e carrying the beak-like pair of 

 jaws which project from the finely papillate buccal membrane. 

 Three tentaculiferous lobes of the fore-foot are in immediate 

 contact with this buccal cone ; they are the right and left 

 (e, c) inner lobes, as we propose to call them, and the in- 

 ferior inner lobe (<t), called inferior because it really lies 

 ventralwards of the mouth. This inner inferior lobe is 

 clearly a double one, representing a right and left inner 

 inferior lobe fused into one. A lamellated organ on its sur- 

 face, probably olfactory in function (),marks the separation 

 of the constituent halves of this double lobe. Each half 

 carries a group of fourteen tentacles. The right and the 

 left inner lobes (c, c) each carry twelve tentacles. Ex- 



FIG. 105. Diagram to show the relations of the heart in the Kollnsca (from 

 Gegenbaur). A. Part of the dorsal vascular tronk and transverse trunks of 

 a worm. B. Ventricle and auricles of Nautilus. C. Of a Lamellibranch, of 

 Chiton, or of Loligo. D. Of Octopus. E. Of a Gastropod, a, auricle ; r, 

 ventricle ; of, arteria cephalica (aorta) ; ai, arteria abdominalis. The arrows 

 show the direction of the blood-current. 



ternal to these three lobes the muscular substance of the 

 mouth-embracing foot is raised into a wide ring, which 

 becomes especially thick and large in the dorsal region 

 where it is notably modified in form, offering a concavity 

 into which the coil of the shell is received, and furnish- 

 ing a protective roof to the retracted mass of tentacles. 

 This part of the external annular lobe of the fore-foot is 

 called the "hood" (figs. 90, 91, m.). The median antero- 

 posterior line traversing this hood exactly corresponds to 

 the line of concrescence of the two halves of the fore-foot, 

 which primitively grew forward one on each side of the 

 head, and finally fused together along this line in front of 

 the mouth. The tentacles carried by the great annular 

 lobe are nineteen on each side, thirty-eight in alL They 

 are somewhat larger than the tentacles carried on the three 

 inner lobes. The dorsalmost pair of tentacles (marked 

 g in fig. 88) are the only ones which actually belong to 

 that part of the disc which forms the great dorsal hood m. 

 The hood is, in fact, to a large extent formed by the enlarged 

 sheaths of these two tentacles. In the Ammonites (fossil 

 Tetrabranchiata allied to Nautilus) the dorsal surface of 

 the hood secreted a shelly plate in two pieces, known to 

 palaeontologists as Trigonellites and Aptychus. Possibly, 

 however, this double plate was carried on the surface of 

 the bilobed nidamental gland with the form and sculptur- 

 ing of which, in Xautilus, it closely agrees. All the ten- 

 tacles of the circum-oral disc are set in remarkable tubular 

 sheaths, into which they can be drawn. The sheaths of 

 some of those belonging to the external or annular lobe are 

 seen in fig. 91, marked n. The sheaths are muscular as 

 well as the tentacles, and are simply tubes from the base 

 of which the solid tentacle grows. The functional signifi- 

 cance of this sheathing arrangement is as obscure as its 

 morphological origin. With reference to the latter, it 

 appears highly probable that the tubular sheath represents 

 the cup of a sucker such as is found on the fore-foot of the 



Dibranchiata. In any case, it seems to the writer impos- 

 sible to doubt that each tentacle, and its sheath on a lobe 

 of the circum-oral disc of Nautilus, corresponds to a sucker 

 on such a lobe of a Dibranchiate. Keferstein follows Owen 

 in strongly opposing this identification, and in regarding 

 such tentacle as the equivalent of a whole lobe or arm of a 

 Decapod or Octopod Dibraneh. We find in the details of 

 these structures, especially in the facts concerning the 

 hectocotylus and spadix, the most conclusive reasons for 

 dissenting from Owen's view. We have so far enumer- 

 ated in the female Xautilus ninety tentacles. Four more 

 remain which have a very peculiar position, and almost 

 lead to the suggestion that the eye itself is a modified 

 tentacle. These remaining tentacles are placed one above 

 (before) and one below (behind) each eye, and bring up 

 the total to ninety-four (fig. 91, r, v). They must be con- 

 sidered as also belonging to the fore-foot which thus sur- 

 rounds the eye. 



In the adult male Xautilus we find the following im- 

 portant differences in the tentaculiferous disc as compared 

 with the female (see upper drawing in fig. 88). The 

 inner inferior lobe is rudimentary, and carries no tentacles. 

 It is represented by three groups of lamellae (d), which are 

 not fully exposed in the drawing. The right and left inner 

 lobes are subdivided each into two portions. The right 

 shows a larger portion carrying eight tentacles, and smaller 

 detached groups (q) of four tentacles, of which three have 

 their sheaths united whilst one stands alone. These four 

 tentacles may be called the " anti-spadix." The left inner 

 lobe shows a similar larger portion carrying eight tentacles, 

 and a curious conical body in front of it corresponding to 

 the anti-spadix. This is the " spadix " of Van der Hoeven 

 (36). It carries no tentacles, but is terminated by imbri- 

 cated lamellae. These lamellae appear to represent the four 

 tentacles of the anti-spadix of the right internal lobe, and 

 are generally regarded as corresponding to that modification 

 of the sucker-bearing arms of male Dibranchiate Siphono- 

 pods to which the name " hectocotylus " is applied. The 

 spadix is in fact the hectocotylized portion of the fore- 

 foot of the male Xautilus. The hectocotylized arm or lobe 

 of male Dibranchiata is connected with the process of copu- 

 lation, and in the male Xautilus the spadix has probably a 

 similar significance, though it is not possible to suggest 

 how it acts in this relation. It is important to observe 

 that the modification of the fore-foot in the male as com- 

 pared with the female Xautilus is not confined to the 

 existence of the spadix. The anti-spadix and the reduction 

 of the inner inferior lobe are also male peculiarities. The 

 external annular lobe in the male does not differ from that 

 of the female ; it carries nineteen tentacles on each side. 

 The four ophthalmic tentacles are also present. Thus in 

 the male Xautilus we find altogether sixty-two tentacles, 

 the thirty-two additional tentacles of the female being repre- 

 sented by lamelliform structures. 



If we now compare the fore-foot of the Dibranchiata with 

 that of Xautilus, we find in the first place a more simple 

 arrangement of its lobes, which are either four or five pairs 

 of tapering processes (called " arms ") arranged in a series 

 around the buccal cone, and a substitution of suckers for 

 tentacles on the surface of these lobes (figs. 92, 95, 96). 

 The most dorsally-placed pair of arms, corresponding to the 

 two sides of the hood of Xautilus, are in reality the most 

 anterior (see fig. 75, (6) ), and are termed the first pair. In 

 the Octopoda there are four pairs of these arms (figs. 94, 

 95), in the Decapoda five pairs, of which the fourth is 

 greatly elongated (figs. 92, 93). In Sepia and other Deca- 

 poda (not all) each of these long arms is withdrawn into a 

 pouch beside the head, and is only ejected for the purpose 

 of prehension. The figures referred to show some of the 

 variations in form which these arms may assume. In the 



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