1911] Watson: The Genus Gyrocotyle. 431 



F. SUMMARY. 



1. The genus Gyrocotyle is composed of the following species : 

 G. rugosa, G. urna, G. nigrosetosa, G. fimbriata. These are dis- 

 tinguished on the basis of the following specific characteristics : 



(1) Character of folds of terminal rosette. 



(2) The ratio between the distance from the opening of the 

 uterus to the tip of the acetabulum, and the distance from 

 the opening of the uterus to the level of the opening of the 

 penis. 



(3) Character of lateral frills. 



(4) Presence and distribution of spines. 



(5) Size of tail-rosette. 



(6) Presence of hooked embryo in uterine eggs. 



(7) Presence of an eversible cirrus, adapted to self -impreg- 

 nation. 



(8) Spinules lining ejaculatory duct. 



(9) Operculated uterine eggs. 



2. The normal habitat of the sexually mature individual is 

 in the spiral valve of the intestine of some species of the family 

 Chimaeridae. Reported occurrences of G. rugosa in bivalve 

 molluscs are probably accidental. Nothing is known of inter- 

 mediate host or life-cycle of the parasite. 



3. The functional orientation of Gyrocotyle fimbriata directs 

 the acetabulum anteriorly, the rosette posteriorly. This is in 

 agreement with Wagener's observations on the living G. urna, 

 but in exact opposition to Lonnberg's observations on the same 

 form. The worm is capable of definitely directed locomotion and 

 is very active under favorable conditions. The exploring func- 

 tion of the acetabulum is strongly in evidence. The posterior 

 rosette functions strictly as an organ of attachment. The aceta- 

 bulum never functions as an organ of attachment. 



4. This functional orientation is borne out by evidence from 

 the structure of the central nervous system and by the presence 

 on the margin of invagination of the acetabulum of a pair each 

 of sensory pits and sensory papillae, abundantly innervated by 

 heavy branches from the central nervous system. 



