966 Cc. H. MARTIN. 
In the living Microstoma the nematocysts may be seen to 
lie in vacuoles just under the skin. They may lie singly or 
to the number of three or four together ; very frequently the 
same vacuole encloses nematocysts of different types. I have 
seen cases in which a vacuole enclosed one large barbed 
nematocyst with three small oval nematocysts, and in other 
cases I have seen the barbed form accompanied by the 
cylindrical form. 
In Microstoma lineare from Loch Tay and Loch 
Lomond I have found three kinds of Nematocysts, which 
cannot be distinguished from the nematocysts of Hydra 
rubra occurring in the same regions. (a) The large barbed 
nematocysts measure, in an unexploded condition in balsam 
preparations, 12—1l5, length, 7—9, breadth, and when 
the nematocyst is discharged the length of the thread is 
about 400. There are only three large barbs directed 
backwards at the end of the thickest portion of the thread. 
(6) The small cylindrical nematocysts measure in length 
12 4, in breadth 44, and the length of the thread is about 
300 uw. (c) There are also small spherical nematocysts 
with a thick thread and no barbs. In sections through 
a Microstoma nematocysts may be found lying in four 
different positions—(1) in the lumen of the gut, (2) in 
the cells of the gut, (3) surrounded by cells in the body 
cavity between the gut wall and the ectoderm, (4) in the 
ectoderm. 
(1) In the gut of a Microstoma which has recently eaten 
a Hydra, the nematocysts may still be found lying in the 
tissue of the Hydra in the lumen of the gut. 
(2) As digestion proceeds, the nematocysts are engulfed 
by the cells of the gut, and may be found lying to the number 
of two or three in the same cell. 
(3) At a later period nematocysts can be fund lying just 
outside the gut, sometimes free, but usually surrounded by 
three or four small cells. These cells seem. to be mesen- 
chymatous phagocytes, though I am not quite sure whether it 
is not possible for the cells of the gut itself to become free and 
