THE AMERICAN NATURALIST [Vol. XLIII 



proglottid and are inserted in the cuticula of each surface 

 by numerous branching strands (Fig. 10). He even 

 thought that the spindle cells might be the tendons of 

 these muscles, which, however, is not the case, since Zer- 

 nicke (1895) and others have demonstrated the branched 

 insertions just mentioned. It is my opinion, however, 

 that it is the pull of these muscles and especially of their 

 branched insertions which interweave themselves among 

 all the peripheral tissues of the body (Fig. 10), which 

 has thus distorted all the cellular elements in this region 

 and caused them to assume their characteristic shape 

 and appearance. And this opinion is confirmed by the 

 fact that in the scolex and between the proglottids, where 

 these muscles are weak or absent, the subcuticular cells 

 are not spindle-shaped, but have the form of ordinary 

 parenchyma cells (Leuekart). 



The embryological and larval history of these worms 

 also furnishes arguments against the epithelial theory. 

 The cuticula comes into existence, both in trematodes and 

 cestodes, before the subcuticular cells have differentiated 

 and grows independently of them (Figs. 8 and 11). Its 

 early growth has been well described by Looss for trema- 

 todes in Diplodiscus (1892) and in a considerable number 

 of distomes (1894) and by Koewer (1906) in distomes, 

 and for cestodes by Young (1908) in Cysticercus pisi- 

 formis. These authors show also very conclusively that 

 when the subcuticular cells do finally make their appear- 

 ance it is as differentiations of the embryonic paren- 

 chyma cells and that at no time is anything like an epi- 

 thelium present in the position in which they are 

 found (Fig. 8). 



The moulting of the outer epithelium (ectoderm) in 

 larval trematodes and cestodes has also an important 

 bearing upon this question, inasmuch as in consequence 

 of it the adult worm is entirely composed of tissues de- 

 rived primarily from the interior embryonic cell mass 

 (endoderm or mesenchyme). The subcuticular cells can 

 not consequently be of ectodermal origin and can not be 



