344 JAMES F. GEMMILL 
General. 
As far back as 1865 A. Agassiz (1, p. 130) inferred from the 
results of tow-nettings that the eight-rayed condition in 
Melicertum campanula was reached by the formation 
of four new interradial outgrowths from the stomach in an 
originally four-rayed young medusa. 
Mayer (7, p. 208) thinks that Melicertum campanula 
(Agassiz) and Melicertidium octocostatum (Sars) are 
probably identical species, and that Melicertum should have 
priority as the generic name. However, there are sufficient 
reasons (especially under (1) and (2) in the following com- 
parison) for keeping Melicertum and Melicertidium as distinct 
genera, at least in the meantime. 
Melicertum 
(hydroid) 
(1) Tentacles united at their 
bases by a membrane. 
(2) A small theea at base of 
hydranth. 
(3) Tentacles up to ten im 
number. 
Melicertum 
(medusa). 
(4) Harliest free stage with 
only two marginal ten- 
tacles. 
(5) No ‘radiating lines’ on 
sub-umbrellar surface. 
(6) Marginal tentacles, in adult 
equal or sub-equal in 
size. 
Melicertidium 
(hy droid) 
(1) Tentacles not united at 
their bases by a mem- 
brane. 
(2) No theca. 
(3) Tentacles sixteen or more 
(up to thirty-two) in 
number. 
Melicertidium 
(medusa). 
(4) Earliest free stage with 
four marginal tentacles 
and four intervening ten- 
tacle buds. 
(5) Numerous ‘radiating lines’ 
onsub-wmbrellar surface. 
(6) Marginal tentacles in the 
largest specimen exam- 
ined consist of about sixty 
small tentacles and about 
eighty much larger ones. 
