Monthly Micrnscopican 
Journal, March 1, 1869. J 
Triarthra longiseta. 
177 
under its edge spring four other pairs ; all are striated, — the strise, 
which are frequently obHque (Fig. 7), averaging 2500 to the inch : 
only one of each pair is represented in the figure. One pair (h) is 
fastened to the dorsal surface : the next (e) consists of from eight to 
ten parallel fibrillse, divided into two main groups, and stretches 
nearly the whole length of the rotifer. The other two (d, e) run 
parallel to the ventral surface ; and the whole four pairs act to- 
gether with surprising vigour, and enable the animal to draw its 
head suddenly in, and so jerk forward the spines. In Pritchard's 
* Infusoria,' Triarthra is drawn in an impossible attitude, for the 
head is represented protruded while the spines are advanced. The 
spines recover their normal position by their own elasticity ; for they 
appear to be hollow at their bases like quills, with one half cut away 
(Fig. 5) where they are fastened to the body. They are also notched 
here and there, and, especially towards the ends (Fig. 6), are occa- 
sionally roughened by minute imbrications. Five circular muscles 
surround the body, and by their contractions force out the retracted 
head. 
The vascular system of Triarthra is delicately transparent, and 
very difficult to be seen. It exists, however, as a long, tortuous 
double thread, or tube, which springs from the cloaca, passes up 
either side of the body close to the surface, and ends in a mesh of 
convolutions below the neck. I could only detect one vibratile tag 
on each side of the neck, and close to the spot from which the 
muscle (c) springs. I have always failed to discover a contractile 
vesicle, but it may be very small, and hidden between the ovary and 
stomach. 
By bringing into focus the central a,nd inner portion of the 
head, a bluish and roughly rhomboidal mass (Fig. 10, a) may be 
seen, above which are placed the eyes, and a prolongation from 
which (h) extends to a fossa beneath the neck bearing long setae. 
From this mass, also, which is probably a cerebral ganglion, may be 
traced, on either side of the body, a curved thread, which ends in a 
rocket-shaped base (c), and Fig. 11, bearing setae. These are doubt- 
less tactile organs ; precisely similar ones exist at the hinder extre- 
mity of Synchoeta tremula. 
Each eye (Fig. 8) is a clear colourless sphere of ^ ^ ih. of an 
inch in diameter, resting on, and partly embedded in, flat plates of 
red pigment. 
Towards the latter end of November, all the specimens I cap- 
tured had winter eggs (Fig. 9), which are of a peculiar shape, and 
are protected by a thick layer of yellowish transparent cells. The 
average length of a full-grown Triarthra is -i-o inch. 
