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. 



Beliind 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 eithei- side 

 an ej/c. The head may be joined to the body by a more or less 

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

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

 or less continuous with llic liead, but ventrally forms a sack 

 anteriorly open, and from wliich emerges the funnel or i<i])}wn 

 f which may also be regarded :is a modification of a portion of the 

 foot of the gasteropoda ). The sacli or body is, in a small portion 

 of theoctopod and in the decapod species, expanded into postero- 

 lateral membranes, possessing the i)Ower of undulatory motion, 

 and which may be considered as the e(|niv:dent of fiii.<, in func- 

 ti<^n. though not in ai)pearanc-e. 



T/u; SHn. 



Tlie e]>illieliuui in tiie (lihr:iuehi:ite ci>pIiMlo]»ods, is composed 

 of flask-like or iouikIcmI gi-ain-like cells; they are cylindrical in 

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

 covers that containing the chromato]ihon'S. 'I'lie skin of the 

 ceplialopod, particularly its dorsal surface, is covered with 

 apparently minute specks of a d;irk reildish color, which are. iu 

 reality, the pigment cells or chromatophoies (V]. 12, f, 8, !)); these 

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

 of the animal, the little sacks are greatly dilated, and the color 

 becomes intensified. The rapid chameleon-like changes of color 

 peculiar to the cei)halo))oda among mollusca, are thus produced ; 

 whilst tile accompanying opal-like and silvery appearance ex- 

 hibited by the cuttle-fishes, is due to a thin l:iyer underlying tlie 

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

 Nautilus are loinid ei)ithelial pigment cells, which, according to 

 Humphius, are used similarly to the chromato])liores. 



The outer skin, in many ol" the genera, is furnished Avith con- 

 tractile tubeicular elevations or beards, which aic laised when 



