LIFE-HISTORY OF MELICERTIDIUM 341 
and ceasing at the base of the hydranth without forming even 
a rudimentary hydrotheca. At no stage are the bases of the 
tentacles united by a web or membrane. The sixteen-tentacled 
stage is entirely similar to young polyps (fig. 15) of the tank 
hydroid deseribed later in this paper, though the latter are 
relatively rather larger, no doubt because they could draw 
during growth on a nutritional reserve greater than was at the 
disposal of the parent of the colony. This year (1919) I have re- 
peated the rearing experiments and obtained the same results. 
Description of the Tank Hydroid. 
In the early spring of 1916, 1917, and 1919 colonies of an 
apparently new theca-less hydroid appeared on stones and 
on glass in several of the tanks at the Millport Biological 
Station. Dr. James Ritchie, Royal Scottish Museum, Hdin- 
burgh, to whom I sent a specimen in February 1917, made the 
conjecture, which has proved right, that it might turn out 
to be the hydroid of some Leptomedusan. A little later in 
the same year young medusae budded off from a colony were 
obtained. They had four radial canals, eight tentacles, no ltho- 
cysts, and no ectodermal ocelli or oral tentacles. I tried to rear 
them, but without success. The matter remained there till 
July 1918, when the results (given above) of rearing Melicerti- 
dium eggs unexpectedly connected the tank hydroid with 
this medusa, and made me undertake more careful experi- 
ments (see below) on rearing the young medusae, when these 
were budded off from the tank colonies in the spring of the pre- 
sent year (1919). The characters of the hydroid are as follows : 
Hydranth: entirely theca-less. Tentacles: long, 
slender, tapering, with solid core of endoderm eells in a single 
row, studded with nematocysts, not united at their bases by 
a membrane, arranged in a single circle but tending when 
fully extended to curve upwards and downwards alternately, 
commonly sixteen in number, but often more numerous 
especially in sterile colonies, in which individuals with as 
many as thirty-two may be noted. Hypostome: conical 
when closed, shaped like a shallow wide-mouthed umn when 
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