428 University of California Publications in Zoology. [Vol. 6 



usually six or eight in number. This ring appears in a rudi- 

 mentary form in the Tetraphyllidea and in the Tetrarhynchidae ; 

 it is well established and clearly marked in the Taeniadae and 

 the Dibothridiata. 



It is at once evident that the nervous system of the rosette 

 in Gyrocotyle is much more easily homologized with the nervous 

 system of the merozoic cestode scolex than is the nervous system 

 of the acetabular region. There are two serious objections to at- 

 tempting to derive the nervous system of the cestode scolex from 

 the acetabular bridge-commissure and its anterior rings. In the 

 first place, the rosette and funnel constitute an efficient organ of 

 attachment, so strikingly like the scolices of many Tetraphyllidea 

 in mode of adhesion and probable developmental history — being 

 formed by partial fusion of the walls of a trough, and later dif- 

 ferentiation of the ends (if (be tube thus formed — that it seems 

 irrational to suppose that two structures of such fundamental 

 similiarity could have been developed independently in two 

 groups of organisms as closely allied as are the merozoic and 

 the monozoic cestodes. The acetabiuum, on the contrary, never 

 functions as a sucker or organ of attachment ; there seems to be 

 no possible relationship between this structure and any of the 

 familiar types of cestode scolex. There is every reason, on the 

 basis of function, derivation and structure for regarding the 

 rosette as a scolex of the phyllidian type. In the second place, 

 there is no "starting-point" for the formation of a ring about 

 the acetabular commissure, no matrix out of which to differentiate 

 the complex rings and connectives of the nervous system of the 

 scolex. Such a matrix is, however, afforded by the numerous 

 anastomosing branches and the two irregular rings and their con- 

 aectives, seen in the rosette extremity. 



These facts, with other considerations previously given, 

 justify the homologizing of the scolex of the merozoic cestode 

 with the rosette of Gyrocotyle, a posteriorly situated organ of 

 attachment. This conclusion implies a functional reversal of the 

 nervous system of the ancestral flatworm in the course of its 

 development into a merozoic cestode. The greatest mass of 

 nervous tissue, cephalized in the primitive flatworm, comes to lie 

 in the posterior region of attachment of the cestode. The anterior 



