ZOOLOGY. 



343 



tagonal and the rays singularly abbreviated and aborted, so tbat tbe 

 form is very unlike an ordinary opbiuroid. In fact tbe arms are for tbe 

 greater partof.tbeir length inclosed in a disk formed of calcareous 

 plates both above and below, but only a small portion of jointed arm 

 projects from each angle of the pentagon thus formed, and, with the 

 structure along the lines of the arms on the lower surface, reminds one 

 of the ophiurids. It is contended that the whole skeleton structure is 

 due to the abnormal development of the ordinary plates of the ophiurid, 

 but at the same time in the structure of the animal a number of charac- 

 ters are observable which tend toward the asteroids, such as a great 

 development of the ambulacral system, with formation of supplementary 

 plates separating the tentacular compartments, the extension of the peri- 

 toneal cavity into the radial portions of the animal, and the organiza- 

 tion of the mouth. 



Mr. Sladen concludes with some general remarks on the characteris- 

 tics and relations of this new type. He argues that " the pentagonal 

 flattened gonio-discoid body, combining within its area representatives 

 of the whole free radial system, as well as of the disk of the typical 

 Ophiuran, presents us with nothing less than the anomaly of an organ- 

 ism having opbiuroid ray-plates expanded and then consolidated along 

 with their disk into a pentagonal asteroid form of test. Such a re- 

 markable arrangement of the entire brachial series and abnormal devel- 

 opment of the side arm-plates is," he says, " so far as lam aware, quite 

 without a parallel amongst the Ophiuroidea, and constitutes a character 

 which I regard as an approach to the structure of the Asteroidea. Al- 

 though it may be asserted that the resemblance is largely superficial, 

 and that the plates, when takeu individually, are truly opbiuroid after 

 all, the objection is counter- balanced by the fact that this combination 

 of radial and interradial systems within a common periphery involves 

 morphologically a principle of much higher import than simply outward 

 form, and to which testimony is borne, more or less fully, by each of the 

 following particulars, that is, (1) " the limitation of the tentacular pore- 

 system to the disk"; (2) "the extremely rudimentary condition and 

 aborted character of that portion of the radial series which is prolonged 

 beyond the body-disk, and which seem to give indications of disuse and 

 cessation of function in this area of Opbiuroid organization, followed by 

 a localization of function according to the plan of Asteroid organiza- 

 tion"; (3) "the extraordinary development of the tentacular or ambul- 

 acral system as compared with its usual standing in Ophiurian anatomy, 

 together with a most extreme modification of the muscular system char- 

 acteristic of that group," indicating unequivocally a tendency toward the 

 growth of Asteroid characters; (4) " the prolongation of the peritoneal 

 cavity into the radial portion of the animal," evincing " a divergence 

 towards a structure usual in the Asteroid gi-oup"; (5) the simple and 

 rudimentary character of the mouth armature," which diflers from the 

 opbiuroid and approximates the asteroid type ; and (C) " the aborted 



