THE NEMATOCYSTS OF TURBELLARIA. 2638 
Von Graff (5), in his great work on Turbellaria in 1882, 
stated that the nematocysts of Microstoma can be distinguished 
from those of Hydra (1) by their smaller size, the length of 
the large capsule measuring 0-015 mm., and the thread 
0:26—0:13 mm.; (2) by the presence of only four barbs. He 
contradicts Hallez, who had correctly described the nemato- 
cysts as lying in vacuoles, asserting that each lies in a single 
cell which (although on this point he seems to have been 
somewhat doubtful) possesses a cnidocil. He also described 
so-called intermediate forms between rhabdites and nemato- 
cysts in Polycystis (Macrorhyncus), Mamertinus, etc., and 
concluded that nematocysts were homologous with rhabdites 
(‘so scheint die auch von Hallez acceptierte Homologie 
zwischen den Rhabditen und Nematocysten der Turbellarien 
ziemlich sicher begrundet, und wir konnen mit M. Miiller 
und lLeuckart die ersteren als niedere Zustande von 
Nesselorgane beobachten ”’). 
Fuhrmann (4) has more recently studied the large oval 
nematocyst in Microstoma. He finds that they measure 
0084—-0187 mm. in length, and that the distal end is closed 
by a lid, which springs off when the nematocysts explode. 
The thread, according to his account, is:solid, measuring 
about ‘137 mm., and there are four long and four short barbs. 
Under these circumstances it would seem very difficult to 
understand how, with a solid thread, the explosion is effected, 
since his drawings show that the barbed base of the thread is 
everted. 
In his latest work on Turbellaria in Bronn’s ‘ Thierreich,’ 
von Graff (7) places all the rhabdite-like structures—rhab- 
_doids, pseudorhabdites, sagittocysts, and nematocysts— 
under a common category of Hyaloids. 
(a) The rhabdoids measure from 0°16 to 87 u, and are 
sub-divided into rhamnites (bent rods) and rhabdites, which 
are more resistent, cylindrical, or elliptical bodies. They may 
be formed either in the mesenchyme or in the epithelial cells. 
(b) Pseudorhabdites are usually confined to the Alloioccela, 
and are characterised by their granular appearance. 
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