CHAPTER IX. 



Family VIII. TRIARTHKAD^. 



'Ro^'Y furnished loith skipinng appendages ; corona transverse ; ciliary wreath single, 

 marginal ; foot absent. 



The four genera wLicli form this family resemble each other in one striking particular. 

 Each 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 moved indirectly by the usual longitudinal muscles ; which, in sharply withdrawing 

 the head, throw the spines forward. In one genus, Ptcroessa, which is known only by 

 its lorica, the spines are very numerous, and are of two distinct patterns ; in another, 

 Polijarthra, 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. Polijarthra 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 tropin. Triarthra hag the malleo-ramate tropin of 

 Melicerta ringens ; in Pedetes the trophi have not been clearly defined ; while Poly- 

 arthra, widely unlike either, has a niastax and trophi closely resembling those of 

 Sijnchceta. Polyarthra , moreover, is still further separated from Pedetes and Triarthra 

 by having one occipital eye, instead of two frontal. 



Genus POLYAETHEA, Ehrenberg. 



GEN. CH. Spines in clusters on the shoulders ; eye single, occipital ; mastax 

 very large and pear-shaped ; tvo'phi 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 Synchmta ; 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 Synchata, 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. PL.iTYrTBEA, Ehrenberg. 



(PI. XIII. fig. 5.) 



Polyarthra plaiyptera^ . . Ehrenberg, Die Jh/iw. 1838, p. 441, Taf. liv. fig. 3. 



Leydig, Uc!). rf. £«!< fZ. iJodcr^/t. 1854, p. 42, Taf. i. fig.,10. 

 „ „ . . Gosse, Phil. Travis. 1856, p. 435, pi. xvii. figs. 44-49. 



. . „ „ „ 1857, p. 320, pi. XV. figs. 27-29. 



„ ,, . . Plate, J■e»ais(;/^. ^ejte./. iVaiwj-. 1885, p. 16, Taf. i. fi;:. 4. 



' Ehrenberg's P. trigla is possibly P. platypiera with the blades seen edgewise. 



J) 2 



