HYDROMEDUSAE. . 729 



into the substance of the umbrella, and the brown pigment of the bulbs is extended along 

 some of the canals. Inside the margin of the umbrella there is a circular band of endoderm 

 cells, but no circular canal can be seen. The groups of nematocysts extend over the ex- 

 umbrella from near the margin to the top of the umbrella and are aiTanged in radial rows, 

 one row between every two tentacles. In each row there are about six to seven circular 

 clusters of nematocysts. Near the margin of the umbrella the canal along which the nema- 

 tocysts travel is present, but it is only visible for a short distance. 



Psythia proUfera, Agassiz and Mayer (1902, p. 143, PI. I.). The authors, in placing the 

 new genus in the family Williadae make the following statement : " In all previously known 

 genera of the family Williadae the radial canals are branched. The general form, colour, shape 

 of proboscis, and method of budding of the present medusa, however, all incline one to place 

 it among the Williadae. It may be a primitive, or ancestral, form in which the canals 

 have remained simple, or possibly an atavistic sport from some of the more complex Williadae, 

 or an immature individual which may give rise to medusa-buds before attaining its complete 

 development." A single specimen was found at the Tortugas, in the Gulf Stream off Florida. 

 The medusa has four radial canals without branches; four tentacles with ba.sal bulbs without 

 any pigment ; four perradial stolons bearing medusa-buds on the side of the stomach. There 

 are no clusters of nematocysts on the ex-umbrella, but the medusa-bud has a few scattered 

 nematocysts on the ex-umbrella, and they are absent in the parent medusa. It is like the 

 first stage of Proboscidactyla gemniifera in possessing four radial canals, four tentacles, and 

 stolons bearing medusa-buds. But the stolons are upon the stomach and not upon the radial 

 canals, as in the other medusa-budding Williadae. The peculiar clusters of nematocysts are 

 absent and also the pigment in the basal bulbs of the tentacles. The presence of medu.sa- 

 buds and the absence of gonads indicate that the medusa is an early stage, but there is 

 no conclusive evidence that it belongs to the Williadae. 



Genus Willia, Forbes, 1846. 

 Generic Character. Williadae with six radial canals leaving the stomach. 



Willia stellata Forbes, 1846. 

 Medusoid form. 



Willsiu stellata, Forbes (1846), (1848, p. 19, PI. I.); Gosse (1853, p. 359, PL XX.); 

 Willia stellata, L. Agassiz (1862); Haeckel (1879); Willsia curnubica, Peach (1867, p. 357, PL I.); 

 Lar sabellarum, Browne (1896, p. 468, PL XVI.); (1898, p. 818, figs. 1—9). 



Hydroid form. 



Lar sabellarum, Gosse (1857, p. 113, PL XX.); Hincks (1872, p. 313, PL XIX.); Allman (1872). 



Distribution. North Atlantic; British Isles. 



Willia mutabilis Browne, 1902. 



Willia mutabilis, Browne (1902, p. 280). 



Distribution. South Atlantic; Falkland Islands (Vallentin). 



