MUSCLES, ARMS AND FINS IN THE CEPHALOPODA. 63 



and very thin. In Sepia the membrane part is covered with a 

 thick sldn which extends beyond it. The firmness of the fins 

 seems to be in direct relation to the habits of the species ; 

 thus the pelagic genera, encountered only on the high seas and 

 possessing the power of darting to some height above the water, 

 are furnished with the most coriaceous fins ; whilst those of the 

 littoral genera arp of a softer consistence. Whilst these natator}^ 

 lobes are of secondarj^ importance as means of locomotion, they 

 serve additionally as a parachute to preserve the position of the 

 body in the water, and to vary the same according to the 

 desire of the animal ; their rapid undulation, commencing from 

 the front or hind part, according to the direction in which the 

 animal wishes to progress, is of course, of considerable aid in 

 navigation. 



The arms (xxvii, 48), are at once organs of locomotion, either 

 by swimming or crawling, of touch and of prehension. In the 

 tetrabranchiates they are multiplied in number but reduced in 

 size and strength, being short, cylindrical, without cupules or 

 sucking-disks, and retractile into distinct sacks (iv, 62) ; in the 

 dibranchiates they are of definite number, namely eight sessile 

 or non-retractile arms ; with the addition of two, generallj^ much 

 longer, retractile, tentacular ar-ms (xxvii, 48) in some of the 

 genera ; and these are all provided with suckers or organs ot 

 prehension. 



The arms of the octopods (xxiii, 1) are longer, more fleshy 

 and altogether better adapted to their creeping locomotion, and 

 to reaching out from their rocky hiding-places to seize the passing 

 prey ; whilst the comparatively shorter arms of the decapods 

 are compensated by the two generally very long, retractile 

 tentacles, the swimming membrane, the more cylindrical narrow 

 body, and the stiifening of the cuttle-bone or pen, in adapting 

 them for their pelagic life. 



The internal face of the arms is provided with sucking-disks 

 or cups (xxiii, 4) intended to retain objects with which they 

 may be brought in contact. The cups are sessile and fleshy only 

 in the octopods, and they are pedunculated and then furnished 

 with an internal corneous ring; armed with a serrated edge (xxv, 

 22), or with a corneous hook in the decapods (xxvii, 43). 



In Eledone and Cirroteuthis the sessile cupules occupy a 

 single median line on the arms, whilst in the other octopod 

 genera they are in two or more parallel lines. In Octopus they 

 are infundibuliform, shallow, with a depressed radiated surface. 

 In Argonauta these cups are slightly narrowed at their base, and 

 in Philonexis (Tremoctopus) they are cylindrical and extensible. 

 The sessile suckers are powerful means of prehension ; they act 

 like cupping-glasses by the withdrawal of a central plug. 



The pedunculated cupules of the decapods occupy alternately. 



