ON MQRISIA TYONSI. 369 
The two barbless kinds of nematocysts seem to occur chiefly 
on the ocellar bulbs and on the tentacles ; as in the hydroid, 
the smaller pip-shaped kind was found in great abundance ; 
the cylindrical form, on the other hand, was very rare. 
The nematocysts of the hydroid and of the medusa were 
carefully examined and compared at the suggestion of Prof. 
Minchin; it seemed of great interest to find out to what 
extent these structures correspond in the two stages of a 
Hydromedusan. This is a subject which has been much 
neglected by students of the group, and I am not aware that 
any such comparison has previously been made. The fact 
that the nematocysts are identical in both stages of Mcerisia 
is somewhat important, and it would be interesting to know 
whether this is also the case in other genera. In the Hydro- 
meduse so little is known about the connection between the 
two stages and the assignment of medusz to hydroids is often 
only a matter of inference; if it could be proved that the 
nematocysts are as a rule identical in the medusa and hydroid 
of the same species we would have a most excellent test to 
apply, and. many interesting problems might be solved in 
this way. 
B. Variation. 
Dr. Cunnington and I were fortunately able to collect and 
preserve a large number of the medusz, and on examination 
there proved to be much variation in the number of radial 
canals, gonads, and tentacles. I have counted these organs 
in 400 individuals, and of these 55, or nearly 14 per cent., 
differed from the type described above. 
~ Of these abnormal specimens, eleven exhibited variation in 
the general symmetry of the medusa; one possessed only 
three radial canals, three gonads and three tentacles, nine had 
five, and one six of these structures. . 
This type of meristic variation is known to occur frequently 
among Medusze, and I must refer the reader to the papers of 
Agassiz (9) and Browne (10) for further information on this 
subject. 
