MEDUSA. I. 



irregular manner, all branches communicating with the circular vessel. The gonads are developed upon a number of the radial 

 canals adjacent to the stomach. More than 100 marginal tentacles, each carrying an ocellus on the inner side of the bulbous 

 base. Velum is well developed. 



North-Atlantic coasts of Europe and off Newfoundland. 



Family Mitrocomidae Torrey. 



Leptomedusae with open marginal vesicles. 



The first author who has paid attention to the- systematical importance of the open marginal 

 vesicles is E. Metschnikoff (1886 a, p. 5: 1886 b, pp. 81 ff.). He separated Halopsis ocellata Agassiz 

 from the squorid<p (among which it had been placed by A. Agassiz and Haeckel) and the genera 

 Tiaropsis and Mitrocoma from the Ezwopidce, and united the said forms into a family Lafocida. 

 Metschnikoff had demonstrated that the planulse of "Laodice cruciata" as well as of Mitrocoma 

 anna developed into hydroids, exactly resembling Cuspidella Hincks. At the first sight this seems 

 rather peculiar; but Metschnikoff calls attention to the fact that Laodice is an Ocellate, Mitrocoma 

 a Vesiculate, while Tiaropsis forms the connecting link between the two. If we regard Tiaropsis as a 

 more primitive form of the Lafoeida, the Thaumantida (to which belongs Laodice) and the medusae 

 with open marginal vesicles have to be regarded as two diverging branches of the same group. 



Ma as (1893, p. 60) amends the family Lafoeidce sensu Metschnikoff, including Tiaropsis, Mitro- 

 coma, Pkialis (i. e. Halopsis cruciata Agassiz), Halopsis ocellata, and perhaps Euchilota and Mitro- 

 comella. 



Torrey (1909, p. 16) proposes the name of Mitrocomidce for this family, because the medusse 

 in question bear no relation to the hydroid-family Lafoeid(z. The name Mitrocomidce is also used by 

 Browne (1910, p. 32), who gives a revision of the genera of the group and announces a critical revi- 

 sion of the species. Browne hesitates to refer Halopsis to this family, until its marginal vesicles 

 have been thoroughly examined. Later Bigelow (19143, p. 102) has demonstrated that Halopsis 

 ocellata has open sensory x pits of the type of the Mitrocomidce. Bigelow (1913) also unites the 

 leptomedusa^ with open marginal vesicles into a family Mitrocomida, whereas Mayer (1910) does 

 not apply more systematical importance to the open marginal vesicles than that of a generic 

 character. In the quoted paper (1913) Bigelow demonstrates that "Laodice celhtlaricf A. Agassiz has 

 open marginal vesicles and accordingly belongs to the Mitrocomidtx. As this species has many margin- 

 al vesicles but is destitute of cirri, it makes a proper genus, Halistaura nov. - - As generic charac- 

 ters Browne uses the presence or absence of cirri or ocelli together with the number of marginal 

 vesicles. Thus the genus Mitrocomella is only separated from Mitrocoma by the number of vesicles 

 being constantly 16, whereas in the full-grown Mitrocoma the number exceeds 16. This does not seem 

 to me to be sufficient reason for a distinction of genera. The species polydiademata (the only species 

 of Mitrocomella hitherto known) does not differ from the species of the genus Mitrocoma in any im- 

 portant characters, and I prefer, therefore, to refer it to that genus, following Mayer (1910, p. 290). 



A synopsis of the genera of the family Mitrocomidce will now look as represented in Table VI: 



