INTEGUMENT OE VERMES. 139 



morphologically the same as the chitinous carapace of the Arthro- 

 poda, although it is not so hard. 



The dermal carapace of the Rotatoria resembles completely the 

 chitinous skeleton of the Arthropoda. Although it may not become 

 as strong, yet the rigidity of the most anterior segment, as well as of 

 the succeeding ones, which are connected together by softer inter- 

 mediate pieces, gives to it the character of a true skeleton, which 

 serves for the origin of muscles. 



The cells of the Bryozoa are also cuticular structures ; they are 

 sometimes gelatinous (Lophopus crystallinus), soft and flexible ; 

 sometimes, owing to calcareous deposits, they are much harder. 

 The latter kind are found in most of the Gymnok-emata. They differ 

 from the tubes of many Rotatoria, and of tubicolous Annelids, by 

 their close connection with the body ; whereas in Rotatoria and 

 Annelids the tubes are formed by a secretion which is detached from 

 the surface of the body. But the fact that in many Rotatoria the 

 body-wall loses its connection with the hinder portion of the tube, 

 shows that there is no well-defined boundary between these struc- 

 tures. We see, in fact, that there are intermediate steps between 

 typical cuticular structures and other secretions, which are ordinarily, 

 though wrongly, grouped in contrast with them. 



The firm cell of the Bryozoon is not developed over the whole 

 body. It only surrounds the hinder portion, and is continued into a 

 more delicate chitinous layer, which invests the anterior tentacle- 

 bearing portion, but is frequently wanting. This difference in the 

 differentiation of the integument leads to a difference in the power of 

 movement of these two regions of the body, and allows the anterior 

 portion to be retracted, and to hide itself and its crown of tentacles 

 in the hinder or cell-bearing portion. There are various differentia- 

 tions of the cell, which, more or less, bring this relation of the parts 

 to a state of perfection. 



§ 109. 



Those special structures, the aciculi, setee, hooks, and so on, 

 which often play an important part in the economy of the animal, 

 must be regarded as differentiations of the integument, of the class 

 of cuticular formations. The structures in question are most extra- 

 ordinarily varied in character, and may be divided into two groups, 

 according to their relations to the surface of the body. In the first 

 group they have the character of simple elevations of the integument. 

 A thicker cuticular layer is formed on papilliform processes, which 

 may take the form of a wart, or when elongated, of a hair or seta. 

 Even when it is very firm, it is still only apparently an indepen- 

 dent structure, for it is nothing more than a modification of the 

 cuticle, into which it passes at its base. The firm papillas and 

 aciculi which are found in the skin of many Trematoda, and some- 

 times variously extended over the anterior region of the body, are 

 organs of this kind. So too are the fine and closely approximated 

 aciculi, which cover the body of the Solenogastres as far as the 



