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THE AMERICAN NATURALIST [Vol. XLIII 



never have an outer epithelium, but the characteristic 

 cuticula is their earliest body-covering. 



Although there can be no doubt that the outer epi- 

 thelium of trematode and cestode embryos and larvas is 

 often, perhaps usually, moulted, it must be mentioned 

 that cases have also been recorded in which no such 

 moulting has apparently taken place. Schauinsland 

 (1883) has shown that in the embryo of Distomum tereti- 

 colle, the ectoderm gradually loses its cell boundaries and 

 nuclei and becomes metamorphosed into a cuticula. A 

 similar process has been described by Leuckart (1886) in 

 the young redia of Fasciola hepatica, 

 by Zeller (1872) in the embryo of Poly- 

 somia, and by other authors. 



These facts and others which will be 

 mentioned have led many helminthol- 

 ogists to subscribe to a third theory of 

 the cuticula of trematodes and ces- 

 todes— that which sees in it a meta- 

 morphosed or cuticularized epithelium 

 (ectoderm). This is one of the oldest 

 of the theories relating to the cuticula, 

 having been first proposed by Wag- 

 ener in 1855, and in later years having 

 such able supporters as Monticelli, 

 Goto, Nickerson and Braun, although the last named has 

 apparently abandoned it in favor of Blochmann's theory. 

 It is based mainly upon the facts that embryonic and 

 larval ectoderms have been observed in a degenerate 

 condition as just stated, and also that frequentlv nuclei 

 are found imbedded in the adult cuticula. These nuclei 

 are sometimes well formed (Fig. 12), but often have the 

 appearance of being in a more or less broken-down condi- 

 lon and to be degenerating. Open spaces and vesicles are 

 also often present in the cuticula. 



BmuMlSOS l>eei - ° bserved which su PP<>rts this theory, 

 cuticula of Monostomum mutabile, Maclaren (1905) found 



