ON MGRISIA LYONST. 359 
Martin who very kindly assisted me in making out the 
structure of the different forms of nematocysts. 
To Professor Sir Ray Lankester I wish to convey my 
thanks for the interest he has shown in this publication, and 
for allowing me to engage a skilled artist for the preparation 
of one of my plates. 
I. Tat Hyprorp. 
“A. Anatomy. 
The hydroid stage of Mcerisia was obtained on several 
occasions by means of a light dredge from the surface of the 
mud which covers the lake-bottom. Here at depths varying 
from six to fifteen feet it forms curious tangled-looking 
colonies usually growing on branches of Cordylophora 
(fig. 1). 
The hydrorhiza is inconspicuous and consists of a short 
stolon-like tube of ccenosare invested by a delicate perisare 
which is slightly annulated and has a very dirty appearance, 
due to the inclusion in its substance of particles of mud and 
other foreign matter (fig. 2, Per.). From it are given off, 
indiscriminately in all directions, long, filiform branches 
which bear hydranths at their distal extremities and repre- 
sent the hydrocaulus of the colony. 
The free end of the hydrorhiza usually narrows out to form 
a slender stem also bearing a hydranth. 
The proximal part of the branches are covered by the 
annulated perisarc ; this is continued as a smooth, thin mem- 
brane which loses itself a short distance up; the remainder 
is quite naked. 
The curious appearance of the colonies is due to this want 
of rigidity in the hydranth-bearing stems; the latter may 
attain the comparatively great length of over 1 cm., and 
become entangled both with neighbouring branches and with 
the Cordylophora on which the animal grows. 
The hydranths attain a length of over 2 mm.; they may 
