THE NEMATOCYSTS OF TURBELLARIA. 269 
POLYCLADA. 
Hyporhynecus armatus. 
Anonymus virilis. 
Stylochoplana tarda. 
ALLOIO CELA. 
: Allostoma monotrochum. 
Plessisia setosa. 
Of all the Turbellaria, as far as I can discover—Microstoma 
lineare, Stenostoma sieboldii, Plessissia setosa, and 
Stylochoplana tarda are the only forms in which the 
actual expulsion of a thread from the nematocyst has been 
described. 
In most of the other species of Rhabdoccels the so-called 
nematocysts are bodies of very small size in which the appear- 
ance of a coiled thread has been suspected. 
The two Polyclads in which nematocysts have been 
described are unfortunately very rare Stylochoplana 
tarda and Anonymus virilis having only once been 
found. 
Microstoma lineare.—In Microstoma lineare nema- 
tocysts are by means of universal occurrence. Du Plessis 
(16) described a variety from the deeper water of Lake 
Geneva in which nematocysts were absent. ‘This form has 
more recently been raised to specific rank by Zacharias (20) 
under the name of Microstoma inermis. 
I have twice found a similar form in Loch Tay at a depth 
of about 200 feet. It is characterised by the absence of eyes 
and nematocysts, but as nearly all littoral Rhabdoccels lose 
their eyes in deep water, and as I hope to show that the absence 
of nematocysts is merely the result of the absence of Hydra 
at this depth, I do not think the species can be maintained. 
In those animals in which they occur nematocysts vary 
enormously in number, and it is fairly evident that they are 
most numerous in the forms from Scotch lochs in those 
individuals caught on stones on which Hydra rubra is 
very abundant. 
