LEPTOMEDUS^E — PHIALIDIUM. 



267 



Browne, 1 896, 1903, 1905, has made a careful revision of the species of Pkialidium found 

 off the British coast, basing his conclusions upon the study of living specimens. He finds that 

 it is highly probable that Thaumantias hemisphterica, of Forbes and others, is in reality a 

 Pkialidium, which he calls P. temper arium. I think it would be more conservative not to 

 change the old specific name, and therefore propose to retain the name hemisphcericum. 



Browne also finds that the following "species" of Thaumantias described by Forbes are 

 merely stages in the growth of Phialtdium hemisptu 



lurricum: 



Browne describes the adult medusa of P. hemisphcericum as being 21 mm. wide, nearly 

 hemispherical, with thin, gelatinous walls. 30 to 39 tentacles, 1 to 3 lithocysts, usually 2, 

 between each successive pair of tentacles. Each lithocyst is a small, closed sac containing a 

 single spherical concretion. 4 narrow, straight radial-canals, with linear, elongate, folded, 

 oval gonads on their outer halves, not quite touching bell-margin. Stomach short, with 4 

 simple lips, and without a peduncle. Gonads and stomach usually yellowish-brown, tentacle- 

 bulbs brown or reddish-brown. Specimens found by me off Mousehole, Cornwall, England, 

 October and November, 1907, had the tentacle-bulbs, gonads, and stomach green. 



When this medusa is 0.75 mm. high and 0.75 mm. wide there are only 4 tentacles and 4 

 interradial tentacle-bulbs. 8 lithocysts. The 4 gonads are seen as 4 minute vesicles at the 

 middle points of the 4 radial-canals. 



When about 4 to 5 mm. wide the medusa is hemispherical, with 16 tentacles, and 16 or 

 more lithocysts. 



This medusa is exceedingly abundant off the Atlantic coasts of Northwestern Europe, and 

 I believe it to be identical with " Clytia flavidula" Metschnikoff, of the Mediterranean, having 

 seen the Mediterranean medusa alive in numerous surface-tows made at Naples by the Stazione 

 Zoologica, during the winter of 1907-08. P. hemisphcericum is found throughout the year in 

 the Firth of Clyde, Scotland, the young being found from January until April, and the adults 

 from the end of May until autumn, when they are most numerous. The same statements may 

 be made of our American P. languidum off the coast of Massachusetts north of Cape Cod. 



The mature hydroid is not known with certainty, but it is probably a Campanulina (see 

 Hincks, 1868, p. 179). The stem is rooted by a threadlike hydrorhiza, and is simple, ringed at 

 base and just below calycle or sometimes throughout its length. Hydrothecae with 7 to 9 

 denticulations on margin. Reproductive calycles unknown. This hydroid was reared from 

 the egg of the medusa by Wright. 



Calkins, 1899, describes a very similar hydroid from Puget Sound, Pacific coast of North 

 America. 



The early development of P. hemisphcericum = Clytia flavidula, of the Mediterranean, 

 was studied by Metschnikoff", 1 886. He finds that the egg is about 0.26 mm. in diameter and 

 is laid between 8 to 9 in the morning in March and April. Segmentation is total and nearly 

 equal, and the entoderm is formed by polar ingression of cells from the hinder end of the larva. 

 At first the one-layered larva is quite irregular in outline, but later it becomes elongated, 

 spindle-shaped, and ciliated. Nematocysts develop in the ectoderm of its hinder end and it 

 finally attaches itself by its forward end, which becomes the hydrorhiza, while the hinder end 

 develops into the polyp. 



The following is from MetschnikofF's description of his ''Clytia flavidula" of the Medi- 

 terranean, and I feel assured that the medusa he refers to is identical with "Thaumantias 

 hemisphcerica" Forbes and Phialidium temporarium Browne: Bell about 15 mm. wide, flat, 



