MEDUSA. I. 67 



the tentacular bulbs are broadly conical; the number of tentacles amounts to at least about 450. 

 Between every successive pair of tentacles there is one long, fine, solid cirrus (more seldom two cirri), 

 which may coil itself spirally. The marginal vesicles are large open folds of the velum, containing a 

 large number of lithocysts. The number of marginal vesicles between two successive radial canals is 

 very variable according to the distance between the points of termination of the canals; the average 

 number is 5 6. The velum is 2.5 3 mm broad. - The specimens, preserved in formaline or alcohol, 

 are colourless. 



The genus Halopsis was established by A. Agassiz (1863 and 1865) as a genus belonging to 

 the family ^Lquorida and containing the species H, ocellata with about 16 radial canals, and H. cru- 

 ciata with 4 radial canals. In Haeckel's System der Medusen (1879) Halopsis ocellata remained among 

 the sEquorida', whereas Halopsis cruciata became the representative of a new genus Phialis of the 

 Eiicopidte. -- As mentioned above (p. 58), Metschnikoff (1886 a, 1886 b) established a new family for 

 Leptomedusae with open marginal vesicles, the family which we now call Mitrocomida, and he referred 

 Halopsis cruciata as well as ocellata to this family. He was followed in this respect by Ma as and 

 Torrey, whereas Browne (1910) thought it more correct to await a closer investigation of the margin- 

 al vesicles of Halopsis ocellata, before it should be definitively included in the family Mitrocomidce. 

 Such an investigation was carried out by Bigelow (1914 a), and the systematical position of the 

 species is thus stated. In Mayer, Medusae of the World (1910) Halopsis ocellata is still placed among 

 the sEquorida. 



A. Agassiz (1865) has given a thorough description of this species. True, the marginal vesic- 

 les are called "large, compound eyes", but the drawings (figs. 146 and 147) leave no doubt but that 







they are identical with the large open marginal vesicles. The North-European specimens, examined 

 by me, agree in every regard with Agassiz's description of the American Halopsis ocellata. Agassiz 

 states, it is true, that the mature individuals are 2 2'/a inches in diameter (= 5065 mm), whereas 

 the European specimens reach maturity when about 35 mm wide and, according to the material 

 hitherto known, do not exceed 56 mm in diameter. It must be remembered, however, that I have only 

 measured preserved specimens, and the size becomes usually somewhat reduced by the preservation. 

 But even though the European specimens do not, as a rule, reach quite the size of the American 

 specimens, the agreement with regard to the shape is so complete that there can be absolutely no 

 doubt, but that they belong to the same species. - - Agassiz has examined young specimens and 

 found that they have only 4 radial canals. 



The species, described by Agassiz, was seen again, for the first time, by Fewkes (1888 a, 

 p. 233), who found individuals up to 6 inches in diameter. His specimens differ from the type by the 

 fact that "the radial canals arise regularly, not in four groups, from the stomach cavity". This may 

 depend, I think, on the fact mentioned above, that the branching of the radial canals takes place and 

 is completed inside the periphery of the stomach, so that the canals are completely separated when 

 leaving the latter. The same "independent origin of the radial canals" has been observed by Bigelow 

 (19143, p. 102), who found four fragmentary specimens of the species. 



The mode of origin of the radial canals was the chief character by which Agassiz would 



9* 



