58 



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 loo marginal tentacles, each carrj-ing 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. 



Leptomedusge 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 occllata Agassiz 

 from W\e. yEquoridcE (among which it had been placed by A. Agassiz and Haeckel) and the genera 

 Tiaropsis and Mitrocoiua from the Encopidcr, and united the said forms into a family LaforidcE. 

 Metscli nikoff had demonstrated that the planulte of ''Laodicc cruciafd'^ as well as of Mitrocoiua 

 anncr developed into hydroids, exactly resembling Ciispidella 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 La/oeidtr^ the Tliaiusiantida- (to which belongs Laodice) and the medusse 

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



Ma as (1893, p. 60) amends the i-a.\\\\\\ Lafoeidir sensu Metschnikoff, including Tiaropsis^ Alitro- 

 con/a, I^liialis (i. e. Halopsis criiciata Agassiz), Halopsis ocellata, and perhaps Eiichilota and Mitro- 

 comclla. 



Torre\' (1909, p. 16) proposes the name of JMitrocoiiiida: for this family, because the medusae 

 in question bear no relation to the hydroid-family Laforida;. The name Miiroconiida' is also used by 

 Browne I1910, 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 familv, until its marginal vesicles 

 have been thoroughly examined. Later Bigelow (1914a, p. 102) has demonstrated that Halopsis 

 occllata has open sensory pits of the type of the Mitrocoinidcc. Bigelow (1913) also unites the 

 leptomeduste with open marginal vesicles into a family Mitrocoiiiidci^ whereas IVIayer (1910) does 

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

 character. In tlie quoted paper (1913) Bigelow demonstrates that '■^Laodice celhtlarid''' A. Agassiz has 

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

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

 ters Browne u.ses the presence or absence of cirri or ocelli together with the number of marginal 

 vesicles. Thus the genus Mitrocomclla is only separated from Mitrocoma bv 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 polydiadeniata (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 Mitrocomidcc will now look as represented in Table VI: 



