1911] Watson: The Genus Gyrocotyle. 425 



uterus in cestodes, the homologue of the female copulatory duct 

 in the Heterocotylea ; and the vagina is the homologue of the 

 heterocotylean uterus. This gives exactly similar anterior and 

 posterior relations to the openings of the ducts, the penis-open- 

 ings being very slightly anterior to the vaginal opening~and the 

 birth-pore lying most posteriorly and some distance to one side 

 of the other two. 



3. Further morphological evidence in support of this orienta- 

 tion may be adduced as follows : In the first place there are two 

 pairs of abundantly innervated antero-lateral sensory areas, com- 

 parable in structure and location to similar areas in planarians 

 and certain heterocotylean trematodes. In the second place the 

 structure of the central nervous system, when compared with 

 that of the heterocotylean trematodes, affords morphological sup- 

 port to this orientation. The anterior commissure, giving off 

 sensory branches, the main and secondary longitudinal nerve 

 strands, the eight posterior branches and a posterior ring com- 

 missure are all common and similarly placed in Gyrocotyle and 

 the heterocotylean, as for example in Tristomum molae, whose 

 nervous system was described by Lang (1882), (pi. 47, fig. 79). 

 There is added in Gyrocotyle the delicate median portion of the 

 bridge commissure and the second ring commissure, which may 

 well have arisen in Gyrocotyle in correlation with the increased 

 mass and complexity of the musculature of the posterior organ 

 of attachment, the rosette. Thus a comparison of the nervous 

 system and the position and innervation of the organ of attach- 

 ment of Gyrocotyle with the heterocotylean compels us to homolo- 

 gize the rosette with the posterior sucker of the trematode. 



The development of two bridge commissures at the two ex- 

 tremities of the body, approximately equal in abundance of 

 ganglion cells but the anterior supplied with a well-developed 

 median part which is very faint in the posterior one, indicates 

 the manner in which the evolution of the nervous system of the 

 merozoic cestode has taken place. This is, briefly, by the de- 

 generation of the anterior commissure associated with the 

 reduction in the locomotor and sensory functions of the animal, 

 and the great development of the posterior commissure and its 



