LEPTOMEDUS.E — MITROCOMA. 



287 



tentacles. Should we attempt to do so it would be necessary to erect a new genus for almost 

 every one of the known species, as has practically been done by Haeckel, 1879. We prefer to 

 place all these forms under one genus on account of their close relationship and resemblances 

 in all respects excepting in the absolute number of the lithocysts and tentacles, allowing these 

 to serve as specific distinctions. 



The cirri in Mitrocoma resemble those of Laodicea in that they arise from the exumbrella at 

 a little distance above the upper nerve-ring, and their cores consist of entodermal cells which are 

 directly continuous with the entoderm of the circular canal (see Maas, 1893, p. 58, taf. 6, fig. 5). 



Mitrocoma is distinguished from Tiaropsis by the absence of ocelli in the entoderm of the 

 circular canal. Also in Tiaropsis there are no cirri. 



Mitrocoma is a good connecting link between the Eucopidae and the Thaumantiada?, for it 

 possesses marginal cirri similar in structure to those of the Thaumantiadae, while the presence ot 

 lithocysts places it among the Eucopidae. Metschnikoff, 1886, points out the close relationship 

 between the hydroids ot Laodicea and of Mitrocoma. 



It seems possible that the genus Halopsis of the ^Equoridae may have arisen from some 

 form of Mitrocoma through the multiplication of the radial-canals. 



Metschnikoff, 1886, reared the larvae produced from the eggs ot Mitrocoma anna of the 

 Mediterranean and they developed into hydroids having the characters of Cuspidella Hincks. 



Fig. 153. — Mitrocoma cirrata (Mitrocomium cirratum), after Haeckel, 1879. 

 Fig. 154. — Mitrocoma cirrata, from life, by the author. Zoological Station, Naples, Dec. 

 A. Lithocyst. R. One of the ectodermal pigment spots on bell-margin. 



1907. 



Mitrocoma anna? Haeckel. 



Mitrocoma anna, Haeckel, 1864, Jena. Zeitsch. fur Naturwissen., Bd. I, p. 332; 1879, Syst. der Medusen, p. 189, taf. 10, fign. 

 1-13. — Metschnikoff, E., 1886, Embryol. Studien an Medusen, Wien, pp. 23 (egg), 37 (segmentation), 54 (formation of 

 entoderm), 75 (development of hydroid), 82 (polypites), taf. 3, fign. 20-33; taf. 4, fign. 1-16. 



Halopsis anna, Metschnikoff, E., 1870, Verhandl. Gesell. Freunde Naturwissen. Moskau, p. 355, taf. 4, fig. 7. 



( ? ?) Halopsis cruciala, Agassiz, A., 1865, North Amer. Acal., p. 102, figs. 151, 152. 



Non Mitrocomium anna, von Lendenfeld, 1884, Proc. Linnean Soc. New South Wales, vol. 9, p. 606. 



Bell flatter than a hemisphere, 30 to 40 mm. wide, with thick gelatinous walls. 60 to 100 

 long, slender tentacles with conical basal bulbs. 200 to 400 marginal cirri between the ten- 

 tacles. 60 to 100 lithocysts, each consisting of an open fold of the velum containing about 20 

 concretions in 2 crescentic rows. Velum narrow. 4 straight, slender radial-canals. Stomach 

 small, 4-sided with 4 recurved lips, having folded margins. The manubrium is only one- 



L I B R A R Y| 





'SfAS'' 



