ON M@RISIA LYONSI. 365 
transverse fission, perhaps analogous with the monodisc 
strobilisation which occurs in some members of the Acelephe 
(e.g. Cotylorhiza). Polydise strobilisation, he thinks, may 
have developed as a later modification. ‘To quote his words : 
‘“Was bei den Tubularien iitber einen langeren Zeitabschnitt 
sich vertheilte das finden wir bei der polydisken Strobilisa- 
tion zeitlich zusammengezogen und in dem Auftreten mehrerer 
Ringfurchen am oralen Pole ausgepragt.” 
Chun’s suggestion seems, therefore, to be a very happy 
one, for in Meerisia we find strobilisation no doubt derived 
from a process of decapitation, and what was probably a 
phenomenon initiated as a method of getting rid of old 
hydranths has developed into a most efficient method of 
asexual reproduction. 
Murbach (5) has observed a somewhat similar form of 
transverse fission in Hypolytus, a curious solitary hydroid 
from Wood’s Holl, Mass., U.S.A. In this form, however, 
the strobilisation occurs at the aboral, free end of the 
unattached stem; the liberated buds develop directly into 
new hydroids. - 
Il. Tae Mepusa. 
A. Anatomy. 
The liberated medusa (figs. 3, 4) has a globular umbrella, 
the height of which is about equal to the diameter; this 
varies in .the different individuals, but never much exceeds 
4 millimetres. 
The umbrella is eminently contractile, and serves as a 
powerful organ of locomotion, the medusa being propelled 
through the water by alternate dilatations and contractions 
of its cavity. | : 
Except for four bright red ocelli situated on the tentacle 
bulbs the medusa is quite colourless and transparent ; when 
swimming about in the water it is almost invisible. 
The umbrella is thin ina young individual, but as the animal 
