THE SKIN. 11 



with the absence of externally apparent differentiation of parts, 

 organs, or members, has caused the expression " Stupid as an 

 oyster "to be applied to them — an undeserved imputation, as 

 we shall find even the attached oyster, one of the simplest 

 organized of the bivalve moUusca, is well furnished with organs 

 suitable not only for the support, but even for the enjoyment of 

 its placid life — hid though most of these are beneath its envel- 

 oping skin or mantle. 



THE SKIN. 



The epithelium in the dibranchiate cephalopods, is composed 

 of flask-like or rounded grain-like ceils ; they are cylindrical 

 in Nautilus. Under this lies a thin fibrous layer, which 

 again covers that containing the chromatophores. The skin 

 of the cephalopod, particularly its dorsal surface, is covered 

 with apparently minute specks of a dark reddish color ; these 

 are pigment cells or chromatophores (iv, 60, 61), each provided 

 with 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 ceph- 

 alopoda among mollusca, are thus produced ; whilst the accom- 

 panying opal-like and silvery appearance exhibited 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 Rumphius,are used 

 similarly to the chromatophores. 



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

 contractile tubercular elevations or beards, which are raised 

 when the animal is irritated, and give it a rather ferocious 

 appearance. These beards are disposed in symmetrical patterns, 

 upon the dorsal surface, and particularly around the eyes, and 

 their number and position form distinctive characters of the 

 species. 



It will be readily understood, from the above, that color is 

 scarcely characteristic of species in the Cephalopoda. The 

 littoral species of Octopus, etc., which ordinarily await their 

 prey, instead of pursuing it like the finned pelagic species, seem 

 to possess and exercise to some extent through their chromat- 

 ophores, a power of color mimicry. 



The body in the Gastropoda is also completely enveloped by 

 an external more or less elastic skin (iii, 44). Its epithelial 

 layer is formed of quadrangular or prismatic cells, which have 

 a distinct nucleus, and occasionally, when long-cylindrical in 

 form, they have a tail-like end beneath, penetrating the cutis. 

 (Possibly these last are epithelial terminations of nerves — hence 

 sense organs.) Externally the epithelium sometimes supports 

 cilia upon the exposed portions of the body. 



