1896.] BRITISH HYDROIDS AND MEDUSE, 471 
measuring in length the growth of the umbrella. Each series 
arises either on or very close to the margin, and as the umbrella 
grows in length so does the distance increase between the margin 
and each series. 
If all the clusters were present, they would form, at the adult 
stage, four rows, one above the other—the uppermost set, with 
six clusters, representing the first stage, and the lowest, with twenty- 
four, belonging to the adult stage. 
Forbes does not mention the clusters of nematocysts in his 
description of the species. _ Gosse has again described the adult, 
and states that five or six clusters are usually present. He gives 
a figure of acluster, and of a nematocyst which has discharged its 
thread. 
Adult Stage. Twenty-four uniform tentacles—The addition of 
a fourth set of six tentacles brings the young medusa to its adult 
stage. The new tentacles develop just like the previous ones, and 
each one occupies a position midway between the tentacles 
belonging to the first and third stages. I have only seen one 
specimen which shows the connexion between the third stage and 
the adult form. In this specimen the third branch does not leave 
the main radial canal like the first two branches, but is given off 
from the second branch (the branch developed in the third stage) 
and runs down between the main radial canal and its second branch 
to the ring-canal, opposite the new bulb. The subsequent growth 
of the umbrella so separates the branches as to give the appearance 
of each canal dividing, about the middle of its course, into four 
branches, each branch running to one of the tentacles placed about 
equal distances apart on the margin of the umbrella. 
The characteristic features of the different stages may be stated 
thus :— 
First Stage.—6 tentacles ; 6 unbranched radial canals ; 6 clusters 
of nematocysts. 
Second Stage.—12 tentacles ; 1 branch to each canal ; 12 clusters 
of nematocysts. 
Third Stage.—18 tentacles ; 2 opposite branches to each canal ; 
18 clusters of nematocysts. 
Adult.—24 tentacles ; 3 branches to each canal; 24 clusters of 
nematocysts. 
Each set of tentacles usually appears about the same time and 
some grow faster than others. Forbes states that some of his 
specimens possessed only twenty tentacles. 
Abnormal Forms.—First Stage. One specimen with seven 
radial canals, tentacles and clusters of nematocysts. Valencia, 
1895. 
Second Stage. One specimen with seven radial canals, each 
with one branch ; fourteen tentacles and fourteen clusters of nemato- 
cysts. Plymouth, 1893. 
It is clear beyond all doubt that the hydroid Lar sabellarum 
