'2*\ MUSCLKS. A II MS AM I'l.N's. 



bead-cartilage and its projections towards the siphon is found 

 :iiiot her important muscle, that of the neck or collar, which 

 resembles tin- 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 xi/t/ion 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 the t'nmn'1 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 t<> 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 

 ;, valve is developed within the funnel preventing the rellux 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 stitlening processes (appareil de 

 resistance) of which we have already spoken. In On\ choteuthis 

 and Ommastrephes. the funnel is lodged in a special cavity in the 

 under side of the head. 



The so-called lins or swimming membranes, waiitiug to nearly 

 all the oetopods and the Naut ili. exist i n 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. Ommastrephes and in ( hiychoteu! his are 

 formed of transverse muscular layers covered with a very thin 

 epidermis, their surface striated by the muscular fibres beneath. 

 These lins an- not contractile, bat 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 lins seems to be in direct 



