1911 J Watson: The Genus Gyrocotylc. 377 



3. Absence of hooked embryos. 



After careful comparison of the available figures and descrip- 

 tions and a study of about forty specimens, ranging in state of 

 preservation from those perfectly fresh, attached to the_ intestine 

 of the host, down to badly disintegrated specimens from various 

 regions of the alimentary tract of decomposing hosts, the writer 

 has found certain of these proposed criteria to be useful and 

 valid. Certain others seem untrustworthy. These specific char- 

 acters may be listed and criticized as follows : 



1. Presence of lateral frills. It seems fairly certain that 

 lateral frills are present in all species of the genus, when the 

 specimen is in a good state of preservation. Presence of the frills 

 is therefore not a basis for specific distinctions. The amplitude 

 of the frills, and their relative width (in terms of total body 

 width) for any given state of contraction or expansion, is how- 

 ever a characteristic of real value in the recognition of species. 

 G. rugosa (pi. 38, fig. 36) has comparatively scanty and narrow 

 frills ; in G. nigrosetosa the frills are deeper and more volu- 

 minous; in G. urna (pi. 47, figs. 84, 85) they are twice the depth 

 of those in G. nigrosetosa and many times as voluminous. In 

 G. fimbriata (pi. 34, fig. 10) they are still more ample and about 

 the same depth as in G. urna. While these differences can be 

 easily seen, the extreme contractility of the animal makes it im- 

 practicable to attempt to use this characteristic as a basis for 

 separation of species. It is a concomitant, rather than a critical 

 characteristic, not available for exact description. 



Diesing's specimen without frills was taken, it will be recalled, 

 from Mactra. According to the best of our knowledge the only 

 host of the adult parasite is some member of the family Chimaer- 

 idae; this specimen, having- been set free from its host in some 

 fashion, probably entered the shell of the mollusk by accident. 

 Certainly not adapted to life as an ectoparasite, processes of 

 disintegration would undoubtedly speedily set in, and my own 

 experiments with Gyrocotyle in culture media show that dete- 

 rioration invariably begins by cutting off of the lateral frills in 

 increasingly deeper layers. 



2. Presence of spines. This is critical only when applied to 

 living specimens and to those which have been preserved in such 



