172 THE MEDUSAE. 



out attempting a correlation with the Atlantic species, in the hope of simpli- 

 fying the course of other students of this perplexing family. 



First let us consider what characters may reasonably be supposed to be 

 of specific importance. 



The number of tentacles is well known to be variable in all species, so 

 much so indeed that Clans ('80) and Browne discard it. I am inclined, 

 however, to think that if employed with caution it may be of some value. 

 Thus in Rhegmatodes tenuis A. Agassiz it is always small (forty-eight to 

 sixty-four) ; in Zygodactyh groeidanclica always much greater (up to four 

 hundred). The proportional numbers of tentacles to canals is also sig- 

 nificant, for although Claus contends that it is so variable as to be of 

 no service yet all recent descriptions show that in Mesonerna pensile 

 there are always many fewer tentacles than canals; in Zygodactyly groen- 

 landica and A. norvegica (Browne, : 03) always many more; while in other 

 species the numbers are more nearly equal. My own studies suggest 

 that the proportion continues fairly constant with growth. A character 

 of <Teat constancy and one not subject to much distortion with contrac- 

 tion, but the importance of which has been overlooked, is the diameter 

 of the stomach. Thus a very small stomach is constantly found in Rhegma- 

 todes tenuis and R.floridanus, a very large one in M. pensile. Finally, Browne 

 (: 03, : 04, : 05 b ) has urged the taxonomic importance of the form of the basal 

 bulbs of the tentacles, and Maas (: 05), though admitting that they are subject 

 to contraction, believes them to be of value. In many species this is un- 

 doubtedly correct, and this discovery is an important advance. In others, 

 however, the bulbs are simply conical, so that they are not by themselves 

 distinctive, though in connection with other characters they may become so. 



We may leave out of consideration Haeckel's ('79) genera Zygocanna, 

 Zygocannota, and Zygocannula, which are somewhat problematic, and have 

 not, so far as I am aware, been observed since first described; and the fol- 

 lowing species, A. eiliata Eschsch., A. eurkodinu Peron and Lesueur, and A. 

 thalassina Peron and Lesueur, which were founded on such unsatisfactory 

 figures and descriptions that identification with actual specimens is hopeless. 

 Fortunately the remaining Pacific forms are all fully described and well 

 figured. They seem to represent at least seven fairly well-defined species. 

 I. A species with very few (twelve to sixteen) tentacles in comparison 

 with a large number (ninety to one hundred) of canals ; with very thick, 

 ;. lenticular gi latinous disc, broad stomach, and tentacular bulbs with 



