130 



MOLLUSCA 



The fore-part of the foot which surrounds the mouth, as 

 in all Cephalopoda, is drawn out into four or five pairs of 

 lobes, sometimes short, but usually elongated and even fili- 



Fig. 84. Fig. 85. 



Fia. 84. Larvse of Pneumodermon (from Balfour, after Gegenbaur). The 

 prae-oral ciliated band of the trochosphere stage (velum) has atrophied. In 

 A three post-oral circlets of cilia are present. The otocysts are seen, and 

 the rudiments of a pair of processes growing from the head. In B the fore- 

 most ciliated ring has disappeared ; the cephalic region is greatly developed, 

 and, as compared with the adult (fig. 85), is large and free ; the pair of hook- 

 bearing processes on each side of the mouth are retractile, probably part of 

 the fore-foot. At the base of the cephalic snout are seen the pair of arm- 

 like processes (fore-foot) provided with suckers, and behind these the broad 

 pteropodial lobes or wing-like fins of the mid-foot. 



Fio. 85. Pmiimadermon violaceum, d'Orb. ; magnified five diameters, a, the 

 sucker-bearing arms ; b, the fins of the mid-foot (in the middle line, between 

 these, is seen the sucker-like median portion of the foot, by means of which 

 the animal can crawl as a Gastropod) ; c, the four branchial processes. (After 

 Keferstein.) 



form. These lobes either carry peculiar sheathed tentacles 

 (Nautilus), or, on the other hand, acetabuliform suckers, which 

 may be associated with claw-like hooks (Dibranchiata). 

 The hind-foot is probably represented by the valve which 

 depends from the inner ___ c 



wall of the siphon in 

 many cases. 



A shell (figs. 89, 100) 

 is very generally present, 

 affording protection to 

 the visceral mass and 

 attachment for muscles. 

 It may be external or en- 

 closed in dorsal upgrow- 

 ing folds of the mantle, 

 which (except in Spirula) 

 close up at an early period 

 of development, so as to 

 form a shut sac in which 

 the shell is secreted. The 



a 



- 86. Clione borealis, L. ; magnified two 

 diameters, postero-ventral aspect, a, the 

 cephalic region carrying a' three pairs of 

 cephalic cones provided each with very nu- 

 merous minute sucker-like processes, and 

 surrounded by a hood-like upgrowth, 

 and b, the more elongated tentacles (the 

 retractile eye-tentacles are not seen, being 

 placed dorsally) ; c, the pteropodial fins ; 

 rf, the median portion of the foot ; o, the 

 anus ; y, the vagina ; z, the penis. (From 

 Owen, after Eschricht.) 



CeR 



ctenidia are well deve- 

 loped as paired gill-plumes, serving as the efficient bran- 

 chial organs (figs. 101, 103, 

 and fig. 2, B). 



The vascular system is 

 very highly developed ; the 

 heart consists of a pair of 

 auricles and a ventricle (figs. 

 104, 105). Branchial hearts 

 are formed on the advehent 

 vessels of the branchiae. It 

 is not known to what extent 

 the minute subdivision of 

 the arteries extends, or 

 whether there is a true 

 capillary system. 



The pericardium is ex- 



Fio. 87. Enlarged diagram of the nerve- 

 centres of Pneumodermon (from Spen- 

 gel, after Spuleyet). CeR, right cere- 

 bral ganglion ; Pl.R, right pleural 

 ganglion ; Pe, right pedal ganglion ; 

 Vis.R., right visceral ganglion ; I'is.L., 

 left visceral ganglion ; cpe, right cere- 

 bro-pedal connective ; cpl, right cere- 

 bro-pleural connective ; Osp., osphra- 

 dium connected by a nerve with the 

 right visceral ganglion. 



tended so as to form a very 

 large sac passing among 

 the viscera dorsal wards and 

 sometimes containing the 

 ovary or testis the viscero- 

 pericardial sac which opens to the exterior either directly 



or through the nephridia. It has no connexion with the 

 vascular system. The nephridia are always paired sacs, 

 the walls of which invest the branchial advehent vessels 

 (figs. 104, 108). They open each by a pore into the viscerc- 



. 1 C 



9 



Fia. 88. Male (upper) and female (lower) specimens of Nautilus pompilivs as 

 seen in the expanded condition, the observer looking down on to the buccal 

 cone e; one-third the natural size linear. The drawings have been made 

 from actual specimens by A. G. Bourne, B.Sc., and serve to show the 

 natural disposition of the tentaculiferous lobes and tentacles of the circum- 

 oral portion of the foot in the living state, as well as the great differences 

 between the two sexes, a, the shell ; b, the miter ring-like expansion (annular 

 lobe) of the circum-oral muscular mass of the fore-foot, carrying nineteen 

 tentacles on each side posteriorly this is enlarged to form the "hood" 

 (marked v in fig. 89 and m. in figs. 90 and 91), giving off the pair of tentacles 

 marked g in the present figure ; c, the right and left inner lobes of the fore- 

 foot, each carrying twelve tentacles in the female, in the male subdivided 

 intoj), the "spadix" or hectocotylns on the left side, and q, the "anti-spadix," 

 a group of four tentacles on the right side, it is thus seen that the subdivided 

 right and left inner lobes of the male correspond to the undivided right and 

 left inner lobes of the female ; rf, the inner inferior lobe of the fore-foot, a 

 bilateral structure in the female carrying two groups, each of fourteen tenta- 

 cles, separated from one another by a lamellated organ , supposed to be 

 olfactory in function in the male the inner inferior lobe of the fore-foot is 

 very much reduced, and has the form of a paired group of lamella: (d in the 

 upper figure); e, the buccal cone, rising from the centre of the three inner lobes, 

 and fringing the protruded calcareous beaks or .jaws with a series of minute 

 papilla: ; /, the tentacles of the outer circum-oral lobe or annular lobe of the 

 fore-foot projecting from their sheaths ; g, the two most posterior tentacles 

 of this series belonging to that part of the annular lobe which forms the 

 hood (m. in figs. 90 and 91) ; i, superior ophthalmic tentacle ; k, inferior 

 ophthalmic tentacle ; I, eye ; m, paired laminated organ on each side of the 



of the left inner lobe of the fore-foot representing four modified tentacles, 

 ei"ht being left unmodified ; q, the anti-spadix (in the male), being four of 

 the twelve tentacles of the right inner lobe of the fore-foot isolated from 

 the remaining eight, and representing on the right side the differentiated 

 spadix of the left side. The four tentacles of the anti-spadix are set, three 

 on one base and one on a separate base. 



There are thus in the female, where they are most numerous, ninety-four 

 tentacles, thirty-eight on the outer annular lobe, four ophthalmic (a pair to 

 each eye), twelve on each of the right and left inner lobes, and twenty-eight 

 on the inner inferior lobe. 



pericardial sac except in Nautilus. The anal aperture is 

 median and raised on a papilla. Jaws (fig. 88, e ) and a lin- 

 gual ribbon (fig. 107) are well developed. The jaws have 

 the form of a pair of powerful beaks, either horny or calcified 

 (Nautilus), and are capable of inflicting severe wounds. 



