TKACHYMEDUS.E — M1CROHYDKA, GOSSEA. 



367 



tractions. 



its nematocysts. 



It is devoid of cilia. Finally one end becomes attached and the other is elevated 

 into an erect position. When fully grown this minute hydroid is only 0.5 mm. high and from 

 0.15 to 0.175 mm - m diameter. The hydroid devours rotifers, which it captures by means oi 

 Browne, 1906, finds that Allman's hydroid, Schizocladium ramosum ( = 



Obelia) gives off fission frustules, the 

 structure of which exactly resembles 

 those of Microhydra ryderi. 



It is possible that the medusa of 

 Microhydra may prove to be closely 

 related to Craspedacusta sowerbn of the 

 lily-tank in Regents Park, London. It 

 differs considerably, however, in its 

 young stage from the young of Craspe- 

 dacusta, for the young Microhydra 

 medusa lacks all trace of lithocysts 

 while the Craspedacusta has at least one 

 lithocyst when set free, but is in other 

 respects far less advanced than the 

 Microhydra medusa. Moreover, the 



Fig. 209. — Microhydra ryderi, after Browne, in Quarterly Journ. 

 Microscop. Sci. Showing enlarged view of one of the tentacles. 



nematocyst clusters upon the tentacles of Craspedacusta are mounted upon prominent 

 papillae, whereas those of Microhydra are flat (see Browne, 1906, plate 37, figs. 2, 3). Potts, 

 1908, gives a detailed account of the feeding habits of the hydroid and the formation of the 

 medusa-bud. 



Genus GOSSEA L. Agassiz, 1862. 



Gossea, Agassiz, L., 1862, Cont. Nat. Hist. U. S., vol. 4, p. 366— Haeckel, 1879, Syst. tier Medusen, p. 251— Browne, 1904, 

 Fauna and Geog. Maldive and Laccadive Archipelagoes, vol. 2, p. 736.— Bigelow, H. B., 1909, Mem. Museum Comp. 

 Zool. at Harvard College, vol. 37, p. 103. 



The type species is Gossea corynetes described from the coast of Devonshire, England, as 

 Thaumantias corynetes by Gosse, 1853. Haeckel describes a later stage of the same medusa 

 under the name "Gossea circinata." 



GENERIC CHARACTERS. 



Petasidae in which the tentacles are grouped into clusters and do not arise at equal inter- 

 vals from the sides of the bell. No centripetal canals. Development unknown. Bigelow 

 finds that the tentacles are solid, not hollow as described by Haeckel. 



Gossea corynetes L. Agassiz. 



Thaumantias corynetes, Gosse, 1853, Naturalist's Rambles Devonshire Coast, p. 407, plate 21. 

 Gossea corynetes, Agassiz, L., 1862, Cont. Nat. Hist. U. S., vol. 4, p. 366. 



Gossea circinata, Haeckel, 1879, Syst. der Medusen, p. 252, taf. 18, fig. 4— Delap, M. and C, 1905, Annual Report Fisheries 

 of Ireland, 1902-03, part 2, appendix 4, p. 14. 



Haeckel describes a medusa under the name Gossea circinata from the Atlantic coast of 

 France, which is probably the fully developed stage of Gossea corynetes (Thaumantias cory- 

 netes) Gosse, from the coast of Devonshire, England. 



Haeckel's medusa has the following characters: Bell somewhat fuller than a hemisphere, 

 evenly rounded, with fairly thick gelatinous substance at the center, but thin at the margin. 

 12 mm. wide, 10 mm. high. There are 24 large, club-shaped tentacles arranged in 8 (4 radial, 

 4 interradial) groups of 3 tentacles each. These are not quite as long as the bell-diameter, 

 are armed with numerous swollen rings of nematocysts. Each tentacle projects inward in a 

 spur-like basal extension into the gelatinous substance of bell above margin. About 8 short, 

 adradial, slender, helically-coiled marginal cirri, which alternate in position with the 8 

 clusters of tentacles. 24 inclosed lithocysts, 3 between each cluster of tentacles. These 

 lithocysts lie above the ring-canal on the inner side of the velum, and each capsule contains 

 a single spherical concretion. Velum wide and muscular. Ring-canal simple. 4 straight, 

 simple radial-canals. Stomach 4-sided, urn-shaped to conical, with 4 recurved lips with 



