Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/693

Rh 1TEROPODA.J MOLLUSCA 665 like processes, either very short and conical (Clio, Eurybia), or lengthy (Pneumodermon, Octopus) ; these may be beset with suckers or hooks, or both. The mid-foot (fig. 75, mf) is expanded into a pair of muscular lobes right and left, which either are used for striking the water like the wings of a butterfly (Pteropoda), or are bent round towards one another so that their free margins meet and constitute a short tube, the siphon or funnel (Siphonopoda). The hind foot is either very small or absent. A distinctive feature of the Cephalopoda is the ABSENCE of anything like the TORSION of the visceral mass seen in the Anisopleurous Gastropoda, although as an exception this torsion occurs in one family (the Limacinidae). The ANUS, although it may be a little displaced from the median line, is (except in Limacinidie) approximately median and posterior. The MANTLE-SKIRT may be aborted (Gymnosomatous Pteropoda) ; when present it is deeply produced posteriorly, forming a large sub-pallial chamber around the anus. As in our schematic Mollusc, by the side of the anus are placed the single or paired apertures of the NEPHRIDIA, the GENITAL APERTURES (paired only in Nau tilus, in female Octopoda, female Ommastrephes, and male Eledone), and the paired CTENIDIA (absent in all Pteropoda). The VISCERAL HUMP or dome is elevated, and may be very much elongated (see fig. 75, (4), (5), (6)) in a direction almost at right angles to the primary horizontal axis (A, P in fig. 75) of the foot. A SHELL is frequently, but not invariably, secreted on the visceral hump and mantle-skirt of Cephalopoda ; but there are both Pteropoda and Siphonopoda devoid of any shell. The shell is usually light in substance or lightened by air-chambers in correlation with the free-swimming habits of the Cephalopoda. It may be external, when it is box-like or boat-like, or internal, when it is plate-like. Very numerous minute pigmented sacs capable of expansion and contraction, and known as CHROMATOPHORES, are usually present in the integument in both branches of the class. The GONADS of both sexes are developed in one individual in some Cephalopoda (Pteropoda), in others the sexes are separate. SENSE-ORGANS, especially the cephalic eyes and the oto- cysts, are very highly developed in the higher Cephalopoda. The osphradia have the typical form and position in the lower forms, but appear to be more or less completely replaced by other olfactory organs in the higher. The normal NERVE-GANGLIA are present, but the connectives are shortened, and the ganglia concentrated and fused in the cephalic region. Large special ganglia (optic, stellate, and supra-buccal) are developed in the higher forms (Siphono poda). The Cephalopoda exhibit a greater range from low to high organization than any other Molluscan class, and hence they are difficult to characterize in regard to several groups of organs ; but they are definitely held together by the existence in all of the encroachment of the fore-foot so as Fig. 76. Fig. 77. Fio. 76. SpmaHs bulimoidrs, Soul., one of the Limacinidse enlarged (from Owen). C C, pteropodial lobes of the mid-foot ; /, operculum carried on the hind-foot ; g, spiral shell. Fio. 77. Operculum of Spirialis enlarged. to surround the head, and by the functionally important BILOBATION OF THE MID-FOOT. a. Cymbulia Peronii, Cuvier (from Owen). C, C, the expanded pteropodial lobes or wing-like fins of the mid-foot. Two very distinct branches of the Cephalopoda are to be recognized : the one, the Pteropnda, more archaic in the condition of its bi- lobed mid-foot, including a number of minute, and in all probability degen erate, oceanic forms of simplified and obscure organization ; the other, the Siphonopoda, con taining the Pearly Nau tilus and the Cuttles, which have for ages (as their fossil remains show) dominated among the in habitants of the sea, be ing more highly gifted in special sense, more varied in movement, more powerful in pro portion to size, and more heavily equipped with destructive weapons of offence than any other marine organisms. Branch a. PTEROPODA. Characters. Cephalopoda in which the mid-region of the foot is (as compared with the Siphonopoda) in its more primitive condition, being relatively largely developed and drawn out into a pair of wing-like muscular lobes (identical with the two halves of the siphon of the Siphon opoda) which are used as paddles (see figs. 76-86). The hind -region of the foot is often aborted, but may carry an operculum (figs. 76, 77). The fore -region of the foot (that embracing the head) is also often rudimentary, but may be drawn out into one or more pairs of tentacles, simulating cephalic tentacles, and provided with suckers (figs. 84, 85). Though the visceral hump is not twisted except in the Limacinidae (fig. 76), there is a very general tendency to one-sided development of the viscera, and of their external apertures (as contrasted with Siphonopoda). The ctenidia are aborted, with the possible exception of the processes (fig. 85, c) at the end of the body of Pneumodermon. The vas cular system resembles that of the Gastropoda. The ne- phridium is a single tubular Fio. IS.StyUola acicula, Rang. sp. en- body corresponding to the JK^MaA^JB right nephridium Of the typi- fold of same ; e, copulatory organ ; h,, t ,-&amp;lt; i- -.r H pointed extremity of the shell ; i, an- Cal pair OI the archl-MollUSC. terior margin of the shell; n, stomach The anal aperture is usually &amp;gt; liver : Hermaphrodite gonad. placed a little to the left of the median line, more rarely to the right. In the Limacinidte it has an exceptional position, owing to the torsion of the visceral mass, as in Anisopleurous Gastropoda, XVI. 84