MUSCLES, ARMS AND FINS. 27 



relation to the habitudes of the species ; thus the pelagic genera, 

 encountered onl}' 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 are of a 

 softer consistence. Whilst the fins are of secondary 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 undula- 

 tion, 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 are at once organs of locomotion, either b}^ 

 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 two series of distinct sacks ; in 

 the dibranchiates they are of definite number, namely eight 

 sessile or non-retractile arms ; with the addition of two, generally 

 much longer, contractile, tentacular arms in some of the genera; 

 and these are all provided with suckers or organs of prehension. 



The arms of the octopods are longer, more fleshy and alto- 

 gether better adapted to their creeping locomotion, and to reach- 

 ing out from their rocky hiding-places to seize the passing prey; 

 whilst the comparatively shorter arms of the decapods are com- 

 pensated by the two, generalty very large, retractile tentacles, the 

 swimming membrane, the more cylindrical narrow body, and the 

 stiffening 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 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 or with a 

 corneous hook in the decapods. 



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 parallel lines. In Octopus they are 

 infundibuliform, shallow, witli a depressed radiated surface. In 

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



