THE SUBCUTICULAR LAYER. 287 



The most conspicuous and important of these elevations are the 

 hooks which are situated on the apex of the Cestode head, and which, 

 are instrumental in fixing the head, and hence the whole worm, to 

 the intestinal wall of the host. The structure and arrangement of 

 these hooks are also of great systematic importance, as we shall see in 

 the course of our exposition. 



The study of the development of these organs shows that not 

 only are all the hooks reducible to the same primitive form, but 

 also that hooks, spines, and little bristles, in fact all the cuticular 

 appendages, are referable to a common type. This primitive form 

 is simply a little cone, seated on a papilliform elevation of the sub- 

 cuticula, and arising from the hardening of its outer surface, which 

 is at other points also sometimes of a firmer character. If these 



FIG. 146. Development of the hooks of Tcenia serrata. 



cones remain small and thin-walled, then we have the common form 

 of hair or spine. When they grow gradually stronger and bend, 

 and when the continued deposition of skeletal substance renders 

 them even harder, then there arise the characteristic forms of the 

 true hooks. Finally and particularly, in the Tcenice they develop 

 a special process on their lower border, and having formed a well- 

 developed basis or root, become the familiar independent and often 

 even moveable hooks. 



The fully developed hook has a very considerable strength, owing 

 to the thickness of its walls, and to the deposition of carbonate of lime. 

 This strength is gradually acquired as the layers are deposited in- 

 ternally, and slowly hardened. In many cases one can detect the 

 successive layers, even in the full-grown hooks, by the lines running 

 down the hook parallel to the outer surface. The thicker the wall 

 the narrower does the internal space become, and in many species it 

 wholly vanishes. 



The Subcuticular Layer. We usually regard the cuticle of animals 

 as the excretion of an epithelial layer of cells, and we often find, lying 

 below it, a tissue of more or less distinct cellular character. Such a 

 subcuticular layer has often been sought for in the Cestodes. I 



