1911] Watson: The Genus Gyrocotyle. 427 



with the delicate median thread of the rosette bridge-commis- 

 sure, all point to its unmistakable homology with the anterior 

 commissure of trematodes, and so with the typical "brain" of 

 the Turbellaria. 



The posterior commissure with its accompaniments of com- 

 plicated rings and connectives has plainly been developed in con- 

 nection with the musculature of the funnel-shaped rosette-scolex. 

 This complex development of nervous structure in connection 

 with a highly developed musculature is strikingly shown 

 throughout the phylum. The ventral nerve stems, in connection 

 with the ventral creeping muscles, become heavier, more pro- 

 fusely branched, than their homologues near the dorsal surface. 

 The great complexity of the nervous system connected with the 

 posterior sucker of the heterocotylean has already been referred 

 to. The development of a posterior commissure, in itself rather 

 weak, but surrounded by a complex system of rings and connec- 

 tives such as is found in the rosette of Gyrocotyle, is exactly 

 what would be expected in connection with the development of a 

 complicated and powerful organ of attachment. 



The comparison of the nervous system of Gyrocotylt with 

 that of the merozoic cestodes shows two main longitudinal stems 

 in both. These are more or less sharply differentiated but always 

 snfficienth clearly marked to be distinguished from the weaker 

 longitudinal stems, of which there are four or eight, correspond- 

 ing to the eight peripheral nerves of Gyrocotyle. Near the free 

 margin of the proglottid, which must be regarded as anterior, 

 these longitudinal stems are connected by a transverse commis- 

 sure, in close connection with the transverse canal of the 

 excretory system. In the scolex there is a heavy ganglionic 

 bridge-commissure, joining the longitudinal stems, lying midway 

 between the dorsal and ventral surfaces of the body. Peripheral 

 to this and in contact with it only at the points where the longi- 

 tudinal stems enter the commissures, is a more or less complete 

 ring-commissure. This ring reaches its most perfect develop- 

 ment in the Taeniadae. but is present in an incomplete form in 

 Ligula, in the Tetrarhynchidae, and in the Tetraphyllidea. 

 Distal to this commissure and ring is found a more or less clearly 

 developed ring joined to the former by numerous connectives. 



