362 H. s. PRATT, 



structure, the product of the connective tissue or muscles it bounds, 

 or both. WooDWORTH (1891) believes the thick basement - membrane 

 in the integument of Phagocata gracilis, a Turbellarian, to be an un- 

 doubted product of the hypodermis. In my own opinion the integ- 

 umentary membrane or cuticula of adult Trematodes is not an ecto- 

 dermal structure, but is the peripheral portion of the paren- 

 chyma, solidified into a thick membrane, which is exposed and be- 

 comes the body - covering of the worm when the cercarian epithelium 

 is shed. It is impossible to deny, however, that in the early stages 

 of the development of the membrane, while the larval epithelium is 

 still present, it cannot be distinguished from a typical basement- 

 membrane. Looss (1894, p. 134) has observed its development from 

 the time of its first appearance in the young Cercaria. At the time 

 when the vacuolated parenchyma begins to develop and to fill the 

 protocoel the membrane makes its appearance "als eine feine, scharfe 

 Linie zwischen Hautmuskelschlauch und der äussern, zelligen Haut. 



Die Linie wird breiter und breiter dass es also schwer ist, zu 



sehen, woher die Haut kommt." This description would seem to point 

 to a clear homology to the basement-membrane of Turbellaria, and if 

 the worm remained in the stage above described, and the cercarian 

 epithelium were never shed, tliere can be no doubt that its subepithe- 

 lial membrane would be called a basement-membrane as a matter of 

 course. Nevertheless, when the cercarian epithelium has been shed, 

 and the membrane in question has become the worm's integumentary 

 covering, it has received modifications of such a peculiar and special 

 nature that such a designation is no longer appropriate or correct. 

 And even before the epithelium is shed, as is the case in the appendix 

 of the worm we are discussing, the peculiar features of the membrane 

 are already determined; it has all the structural features of a Tre- 

 matode cuticula and cannot be called a homologue of the Turbellarian 

 basement-membrane. 



When we inquire what these modifications consist in, and what 

 the peculiar features of this membrane are, which give it such a spe- 

 cial character, we ascertain that they are precisely those which the 

 peculiar needs of worms of such exclusively parasitic habits make ne- 

 cessary; namely, great thickness, a certain toughness of texture, and 

 an unusual power of growth and regeneration. Some Trematodes, 

 for instance, live in the stomachs of voracious, carnivorous animals 

 whose stomach-juices are able to digest bone, teeth, and even the thick, 

 chitinous shells of Insects and Crustaceans : it is evident that in such 



