128 



MOLLUSCA 



like processes, either very short and conical (Clio, Euryliia), 

 or lengthy (Pneumodennon, 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 Limacinidffi) 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 

 NEPHEIDIA, 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-<;AN<;LIA 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. 



Flo. 7G.Sp!rht!ifi ln<li,ii<H<t<'*, Sun]., one of the Limaciniila? enlarged (from 

 Owen). C t\ ptrroj.i.dial lubes of the mid-foot; /, operculum carried on the 

 hind-foot. ; <i, spiral slirll. 



FIG. 77. Operculum of Spirialis enlarged. 



to surround the head, and by the functionally important 



1ULOBATION OF THE MID-FOOT. 



Two very distinct branches of the Cephalopoda are to 

 be recognized : the one, the Pteropoda, 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 



.,-, Flo. 77. Cymlmlia I'ernnii, Cuvier (from 

 With Owen). 



C, C, the expanded pteropodial 

 lobes or wing-like tins of the mid-foot. 



heavily equipped 



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 tentaclrs. 

 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 Pneumodennon. The vas- 

 cular system resembles that 

 of the Gastropoda. The ne- 

 phridium is a single tubular FIO. 78. siyiioia actcvia, Rang. sp. en 



horlv cori-pqnonrlinrr to flip larged (from Owen). C, C, the wing- 

 like lobes of the mid-foot ; d, mediae 



right neiihridiuill of the typi- fold of same ; e, copulatory organ ; h, 



i . * ,-, i -m- ii pointed extremity of the shell ; i, an- 



CaJ pair ot the arclll-JMollusC. {erior margin of tie shell; n, stomach- 



The anal aperture is usually - >' ver ; " hermaphrodite gonad. 

 placed a little to the left of the median line, more rarely 

 to the right, In the Limacinidie it has an exceptional 

 position, owing to the torsion of the visceral mass, as in 

 Anisopleurous Gastropoda. 



