26 MUSCLES, ARMS AND FINS. 



head-cartilage and its projections towartls the siphon is found 

 another important niuscle, that of the neek or collar, which 

 resembles the cartilaginous neck-plate of the dihraneliiata. 

 Other strong muscles arise from the surface <jf tlie two sijjhon 

 cartilages and form an organ more or less comi)letely tul)ular — 

 the siphon — the important means of conducting the resi)ired 

 water when driven out from between the body and mantle by 

 the contraction of the latter, and serving as a swimming organ 

 also, by the same action, performed with greater vehemence. 

 There are found on either side of the internal shell of the 

 dibranchiates, or in a similar i)osition in the mantle of the shell- 

 less Octopus, upon the continuations of the head-cartilage body- 

 muscles, and there are also important muscles of the mantle by 

 Avhich its lower surface may be compressed forcibly in respira- 

 tion or natation. In swimming, the aperture of t\m funnel or 

 siphon is normally directed towards the head, and its discharges 

 cause a series of backward rapid motions, but the animal is able 

 at will to direct the stream to either side, and even to bend the 

 anterior end of the siphon back upon itself to some extent, when 

 it desires to vary the direction of its movement. In some genera 

 a valve is developed within the funnel preventing the reflux of 

 the water. The funnel is entire in the dibranchiates, but cleft in 

 its length in the Nautili ; upon its base is found, in the decapod 

 genera, a portion of the curious stiffening processes (ai)pareil de 

 resistance) of which we have already spoken. In Onychoteuthis 

 and Ommastrei)hes, tlie funnel is lodged in a si)ecial cavity in the 

 under side of the liead. 



The so-called tins or swimming membranes, wanting to nearly 

 all the octoi)ods and the Nautili, exist in all decapods, in their 

 various genera assuming distinctive forms, which may occupy 

 either the whole of the sides of the body or only a ))orti()n 

 thereof, and even extend behind into a sort of tail. These mem- 

 branes in Loligo, Ommastrephes and in Onychoteuthis are 

 formed of transverse muscular layers covered with a very thin 

 ei)idermis, their surface striated by the muscular fibres beneath. 

 These fins are not contractile, but invariable in form ; tliey arc 

 firm and coriaceous, tlieir edges are always entiic and very thin. 

 In Sepia tiic membrane part is covered with a tliick skin which 

 extends beyond it. The firmness of the fins seems to be in direct 



