62 MUSCLES, ARMS AND FINS IN THE CEPHALOPODA. 



and in the comparatively sluggisli littoral genera it is not 

 found at all. 



In the Grastropoda the cartilaginous internal supporting organs 

 are reduced to two, or sometimes four small plates ; they are 

 found in the head, are more or less closely connected with one 

 another, and are surrounded by the pharyngeal muscles. They 

 form the supporting apparatus of the radula and the parts 

 connected with it, and also afford points to which some of the 

 pharyngeal and radular muscles are inserted (ix, 99). 



MUSCLES, ARMS AND FINS IN THE CEPHALOPODA. 



In JSTautilus we distinguish the two adductor muscles, by which 

 the animal attaches itself to the walls of its shell, and which are 

 united by a horny collar ; and within the shell itself we may 

 notice on either side the impressions of these attachments, 

 sunken into the pearly walls. On the outer side of the head-cartilage 

 and its projections towards the siphon is found another important 

 muscle, that of the neck or collar, which resembles the cartilagi- 

 nous 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 (xxiii, 9) — 

 the important means of conducting the respired water when 

 driven out from between the body and mantle b^^ the contraction 

 of the latter, and serving as a swimming organ also, by the same 

 action performed with greater vehemence. In swimming, the 

 aperture of the funnel or siphon is normally directed towards 

 the head, and its discharges cause a series of rapid backward 

 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 (appareil de resistance) already spoken 

 of In Onychoteuthis and Ommastrephes 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 (xxvii, 46) or only a 

 portion thereof, and even extend behind into a sort of tail 

 (xxvii, 44). These membranes in Loligo, Ommastrephes and in 

 Onychoteuthis are formed of transverse muscular layers covered 

 with a very thin epidermis, their surface striated by the muscular 

 fibres beneath. These fins are not contractile, but invariable in 

 form ; they are firm and coriaceous, their edges are always entire 



