6 THE SKIN. 



typical eight arras. In this animal only, the body is contained 

 within, and protected by, an external shell, which compensates 

 to it the loss of offensive and locomotive power possessed by its 

 more highly organized allies. 



Behind the arms, the circle of which may constitute its crown, 

 that major portion of the head is situated which is usually dis- 

 tinctively so designated ; its dorsal aspect exhibits on either side 

 an I't/r. The head may be joined to the body by a more or less 

 constricted neck, or is frequently sessile upon the latter, without 

 intervening constriction. The body, or mantle, is dorsally more 

 or less continuous with the head, but ventrally forms a sack 

 anteriorly open, and from which emerges the funnel or xiftlnnt 

 which may also be regarded as a modification of a portion of the 

 foot of the gasteropoda >. The sack' or body is, in a small portion 

 oftheoctopod and in the decapod species, expanded into postero- 

 lateral membranes, possessing the power of undulatory motion, 

 and which may be considered as the equivalent of ////x, in func- 

 tion, though not in appearance. 



The SHn. 



The epithelium in the dibranchiate cephalopoda, is composed 

 of llask-like or rounded grain-like cells; they are cylindrical in 

 the Nautilus. Under this lies a thin fibrous layer, which again 

 covers that containing the chroinatophores. The skin of the 

 cephalopod, particularly its dorsal surface, is covered with 

 apparently minute specks of a dark reddish color, which are. in 

 reality, tin 1 pigment cells or chroinatophores ( PI. 1 '2, f. X, ll); t hese 

 are each provided with radiating muscles, by which, at the will 

 of the animal, the little sacks are great ly dilated, and the color 

 becomes intensified. The rapid chameleon-like changes of color 

 peculiar to the cephalopoda among inollusc:i. are thus produced ; 

 whilst the accompanying opal-like and silvery appearance ex- 

 hibited by the cuttle-fishes, is due to a thin layer underlying the 

 pigment layer, and reflecting through it. In the tentacles of 

 Nautilus are found epithelial pigment cells, which, according to 

 Uumphius. nre used similarly to the chroinatophores. 



The outer skin, in many of the genera, is furnished with con- 

 tractile tubercular elevations or Iwdrifx. which are raised when 



