CHAPTER IX. 
Family VII. TRIARTHRADA. 
Body furnished with skipping appendages ; corona transverse ; ciliary wreath single, 
marginal ; foot absent. 
The four genera which form this family resemble each other in one striking particular. 
Fach bears spines, or moveable appendages, by means of which the creature can leap 
through the water. These spines have no connection with the body-cavity, though they 
are moyed indirectly by the usual longitudinal muscles; which, in sharply withdrawing 
the head, throw the spines forward. In one genus, Pieroessa, which is known only by 
its lorica, the spines are very numerous, and are of two distinct patterns; in another, 
Polyarthra, they are clusters of blades borne upon the shoulders ; in the remaining two, 
Triarthra and Pedetes, there is only one simple spine on each shoulder, but Triarthra 
carries also a similar spine on the posterior ventral surface. All the genera are more or 
less loricated. In Pedetes the skin bears hard knobs for the attachment of the spines, 
while Triarthra has it stiffened chiefly round the edge below the neck. Polyarthra is 
semi-loricated; the dorsal surface is very tough and there is a still harder shield 
on each side between the dorsal and ventral surfaces. The ventral surface, however, 
is soft and membranous. In all, the longitudinal muscles are highly developed, and 
coarsely striated. 
The genera differ in their trophi. Triarthra has the malleo-ramate trophi of 
Melicerta ringens ; in Pedetes the trophi have not been clearly defined ; while Poly- 
arthra, widely unlike either, has a mastax and trophi closely resembling those of 
Syncheta. Polyarthru, moreover, is still further separated from Pedetes and Triarthra 
by having one occipital eye, instead of two frontal. 
Genus POLYARTHRA, Ehrenberg. 
GEN. CH. Spines in clusters on the shoulders; eye single, occipital ; mastax 
very large and pear-shaped ; trophi forcipate. 
It is not easy to decide in which family the genus Polyarthra should be placed. Its 
mastax and trophi are almost exactly those of Syncheta; its corona bears styligerous 
prominences similar to those of S. pectinata ; its ciliary wreath is marginal and single, 
though not broken up into curves; and, like Syncheta, it possesses but one occipital eye. 
On the other hand its skipping spines naturally place it with Triarthra and Pedetes, 
which genera it further resembles by its lack of foot, by its habit of carrying its eggs, 
and by the partial stiffening of its skin into an imperfect lorica. 
P. pLAtTyprERA, Hhrenberg. 
(Pl. XIII. fig. 5.) 
Polyarthra platyptera ' 2 ; Ehrenberg, Die Infus. 1838, p. 441, Taf. liv. fig. 3. 
” cr 2 < Leydig, Ueb. d. Bau d. Réderth. 1854, p. 42, Taf. i. fig. 10. 
” 5 E : Gosse, Phil. Trans. 1856, p. 435, pl. xvii. figs. 44-49. 
i ¥ Sy Gs yy «1857, p. 320, pl. xv. figs. 27-29. 
” 9 ° - Plate, Jenaisch. Zeits. f. Natur. 1885, p. 16, Taf. i. fig. 4. 
1 Ehrenberg’s P. trigla is possibly P. platyptera with the blades seen edgewise. 
BQ 
