INTEGUMENT OF AETHEOPODA. 249 



the epidermis of other animals. Although in many cases it contains 

 pigment (Crustacea), it is as a rule colourless^ the coloration of the 

 Arthropoda being generally due to the deposition of pigment in the 

 outer chitinous covering. Below this epithelial layer, which is also 

 distinguished as_ the " hypoderm/' there is a layer of connective 

 tissue, but this is, as a rule, slightly developed in comparison with 

 the cuticular layer, and the matrix. 



§ 192. 



When the secreted chitinous layers are very firm they take on a 

 new function, and form a dermal skeleton, which is not only an 

 organ of protection for the organs placed in the body-cavity, but is 

 also an organ of support, and provides points of origin and 

 insertion for the muscles of the body. This relation holds too for 

 the ap]3endages of the body, the integument of which likewise serves 

 as a skeleton for them. 



The formation of large unequal regions of the body produces 

 changes in the dermal skeleton by giving rise to differentiations in 

 it. These are formed by internal processes and continuations of the 

 dermal skeleton, which are principally found in those segments which 

 carry oral or locomotor organs. These processes are very greatly 

 developed on the cephalothoras of the higher Crustacea. But they 

 are not wanting in the other classes. They are fonnd chiefly in the 

 head and thorax in many orders of the Insecta (Coleoptera, Hymen- 

 optera, Orthoptera), where they form a complicated structure 

 known as the " endothorax." These parts often form an organ of 

 support for the nervous system. They are also important as 

 increasing the internal surface of that portion of the dermal skeleton 

 which gives origin to the muscles, and, in more individualised 

 structures, they are closely connected with the differentiation of 

 the musculature. 



The shells, too, which are formed from the chitinous investment 

 of the mantle-folds of many Branchiopoda and Ostracoda, are of 

 importance as skeletal structures ; so too are the shells of the 

 Cirripedia. However different they may be in form or size they 

 have always the same arrangement. Two pairs of ridges or plates 

 surround the entrance into the mantle-cavity and form a movable 

 operculum. The shell-pieces developed in the Balanidi^, but which 

 are rudimentary only in the Lepadidte, gives rise in the former to a 

 continuous rigid cell (Fig. 119,//), the only movable part of which 

 is the operculum (e) which covers over the entrance into the mantle- 

 cavity. 



§ 193. 



Continuations or processes of the integument take on all kinds of 

 forms, such as spines, setae, or hair-like structures ; these are modi- 

 fied in innumerable ways in the Crustacea, Arachnida, and Insecta ; 

 they are sometimes attached closely and immovably to the chitinous 



