Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/695

Rh SIPHOXOPODA.] MOLLUSCA 667 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. FIG. 84. Larva 1 of Pneumorlermon (from Balfour, after Gegenbaur). The prse-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 lias 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. FIG. 85. Pneunwdermon vinlaceum, d Orb. ; magnified five diameters, a, the sucker-bearing arms ; &, 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 ( Xautilus), or, on the other hand, acetabulif orm suckers, which may be associated with claw-like hooks (Dibranchiata). The hind-foot is probably represented by the valve which depends from the inner wall of the siphon in is very generally present, rr a affording protection to the visceral mass and attachment for muscles. It may be external or en- ^ closed in dorsal Upgrow- FIG. 86. Clione borealis, L. ; magnified two ino- foM of thp rmntlp diameters, postero-ventral aspect, a, the mantle, 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 tins ; rf, the median portion of the foot ; o, the anus ; y, the vagina ; z, the penis. Owen, after Eschricht.) (From CcR 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 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 tf c/rf^l C P* auricles and a ventricle (figs. 104, 105). Branchial hearts ^^^^^M-^-R are formed on the advehent vessels of the branchiae. It is not known to what extent v is.L^r&amp;lt;-~ ^-r ~ v is-R. the minute subdivision of the arteries extends, or 1 ^fl whether there is a true fim llirv 5vtpTn Fla 87 ~ Enlar S ed diagram of the nerve- lary S} ST&amp;gt; .m. centres of Pneumodermon (from Spen- pel, after Souleyet). CeR, right cere bral ganglion ; Pl.R, right pleural ganglion; Pe, right pedal ganglion; I is.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 &amp;gt;sp. The pericardium is ex tended so as to form a very large sac passing among the viscera dorsal wards and sometimes containing the ovary or testis the viscero- right visceral ganglion. 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 viscero- 1 f Fio. 8S. Male (upper) and female (lower) specimens of Nautilus pompilius 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 l&amp;gt;een 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 ; 6, the outer ring-like expansion (annular lobe) of the circtim-oral muscular mass of the fore-foot, carrying nineteen tentacles on each side posteriorly this is enlarged to form the &quot;hood&quot; (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 into/), the &quot;spadix&quot; orhectocotyluson the left side, and q, the &quot;anti-spadix,&quot; a group of four tentacles on the right side, it is thus seen that the sulxlivided right and left inner lobes of the male correspond to the undivided right and left inner lobes of the female ; d, 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 n, 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 (&amp;lt;f in the tipper 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 papillae ; /, 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 ; A-, inferior ophthalmic tentacle ; I, eye ; m, paired laminated organ on each side of the base of the inner inferior lobe (rf) of the female, probably olfactory in func tion ; 7i, olfactory lamellse upon the inner inferior lol&amp;gt;e (in the female) ; o, the siphon (mid-foot) ; p, the spadix (in the male), the hectocotylized portion of the left inner lobe of the fore-foot representing four modified tentacles, eight 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.