No. 516] THE CUTICULA OF C EST ODES 



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surface and being flaked off on its outer, it is not a struc- 

 ture which is formed once for all but one which depends 

 on a more or less uniform secretion from the tissues 

 beneath. 



What then is the morphological significance of the cuti- 

 cula of trematodes and cestodes if it is neither a meta- 

 morphosed epithelium nor the product of an underlying 

 hypodermis or of single-celled glands. I believe that it is 

 the peripheral portion of the parenchyma which forms the 

 outer coating of the body after the disappearance of the 

 larval, epithelium (ectoderm), and which has been solidi- 

 fied into a thick membrane by the secretion of cuticular 

 substance from the whole body of the parenchyma. That 

 the entire parenchyma can thus have a secretory function 

 is proved by the formation by it of the fluid with which 

 it is permeated and its vesicles are filled and also of that 

 which fills the cavity of a cysticercus. 



This theory seems to have originated with Leuckart 

 (1886, p. 367). It has been explained and defended at 

 great length by Looss (1893, 1894, also Braun 1893, p. 

 818, note) and subscribed to by Pratt (1898), Cerfontaine 

 (1899) and Young (1908). It is in certain respects an 

 unusual theory, inasmuch as it implies the absence of an 

 integumental covering of ectodermic origin, which is 

 characteristic of the rest of the Metazoa. But the life 

 conditions of trematodes and cestodes are peculiar and 

 unusual. These worms are exclusively parasitic ani- 

 mals, being the only large groups of Metazoa, so far as I 

 recall, of which this is the case, and this parasitic habit 

 is undoubtedly correlated with the disappearance of the 

 larval ectoderm and the formation of the parenchymatous 

 cuticula, as well as other special features of the structure 

 of these worms. 



The most primitive trematodes, the TemnocephalidaB, 

 are an exception to the rest of the group in possessing an 

 integument composed of a cuticula with an underlying 

 hypodermis, although having the typical trematode struc- 

 ture in other respects. These animals are found adhering 



