1896.] BRITISH HYDROIDS AND MEDUS#. 469 
polypites. The reproductive buds are borne in clusters of three or 
four on the upper portion of the body, and when matured detach 
themselves as free medusiform zooids (planoblasts) ; they are desti- 
tute of an ectothecal covering, and are therefore freely exposed to 
the surrounding water. In anearly stage of development the buds 
are much elongated, and take on their hemispherical form as_ they 
approach maturity. The planoblast, at the time of its liberation, is 
almost hemispherical in form; the umbrella is perfectly colourless 
and destitute of thread-cells. The digestive sac or manubrium is 
very mutable in shape; normally it is subcylindrical, and some- 
what swollen at the base, with a slightly lobate mouth. Six radi- 
ating canals traverse the umbrella, terminating on the margin in 
as many oval bulbs of a brownish colour, from which six smooth 
tentacles originate. Both ocelli and lithocysts are wanting; but 
halfway between every two tentacles a minute sac occurs on the 
margin of the umbrella, containing two or three glittering bodies, 
which appear to be thread-cells. The planoblast, when detached, 
bears with it a portion of the peduncle which had formed the bond 
of connexion between it and the parent stock; this survives as a 
somewhat conical process above the base of the manubrium, but 
it is no doubt absorbed after a time. Six is an unusual number for 
the radiating canals ; amongst the British Hydroida it is met with 
only in Clavatella (which has also occasionally four) and in the 
genus Willsia of Forbes.” 
I have every reason for believing that the young medusa of Lar 
is the first stage in the development of the medusa which has 
been described by Forbes as Willsia stellata. 
At Plymouth in September 1893 the medusoid Wilha stellata 
was fairly abundant, and sufficient specimens were obtained to trace 
its connexion with a medusa which exactly corresponds to the de- 
scription and figures given byHincks of the medusa of Lar sabellarum. 
During April and May, 1895, I again met with Willia stellata in 
Valencia Harbour, Ireland, and was able to confirm the observations 
made at Plymouth. 
Forbes first discovered Witllsia stellata at Oban in 1845, and 
dedicated the genus to Dr. Willof Erlangen. Agassiz changed the 
spelling to Willa. 
Forbes has described the adult form of medusa. The early forms 
of 1t may be naturally divided into three distinct stages, according 
to the number of tentacles :— 
First Stage. Siw wniform tentacles. (Plate XVI. fig. 3.)— 
Umbrella, about one millimetre in length and width, varying slightly 
in shape but usually subhemispherical, with a broad velum. The 
manubrium reaches about halfway down the umbrella-cavity. In 
some specimens taken at Valencia the apical stalk (“Stielcanal” of 
Haeckel), which connected the medusa to the hydroid, was still 
visible in the mesoglea of the umbrella, running from the base of 
the stomach nearly to the ex-umbrella. In most specimens the 
apical stalk had disappeared. 
