26 .Mi;S(.'l,KS, ARMS AM) FINS. 



head-cartilage and its projections towards the siphon is found 

 another important muscle, that of the neck or collar, which 

 resembles the cartilaginous neck-plate of the dibranchiata. 

 Other strong muscles arise from the surface of the two siphon 

 cartilages and form an organ more or less completely tubular 

 the siphon the important means of conducting the respired 

 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 position 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 

 which its lower surface may be compressed forcibly in respira- 

 tion or natation. In swimming, the aperture of \\\ufunnel 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 rellux of 

 the water. The funnel is entire in the dib ranch iates, but cleft in 

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

 genera, a portion of the curious stiffening processes (appareil de 

 resistance) of which we have already spoken. In Onychoteuthis 

 and Omniastrephes, the funnel is lodged in a special cavity in the 

 under side of the head. 



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

 all the octopods 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 portion 

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

 branes in Loligo, Omniastrephes and in Onychoteuthis are 

 I'onned of transverse muscular layers covered with a very thin 

 epidermis, their surface striated by the muscular fibres beneath. 

 These tins are not contractile, but invariable in form; they are 

 firm and coriaceous, their edges are always entire and very thin. 

 In Sepia the membrane part is covered with a thick skin which 

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



