482 THE ANATOMY OF INVERTEBRATED ANIMALS. 



opens externally by a pore, and becomes metamorphosed into 

 the ambulacral vessels. But this ambulacral diverticulum 

 does not surround the gullet, and consequently a new mouth 

 is developed in the centre of the ambulacral ring. The larval 

 mouth and gullet are abolished, and the greater part of the 

 body of the Echinopagdium is separated from that portion 

 which contains the stellate Echinoderm. The latter results 

 from the metamorphosis of the mesoderm, which is modeled 

 upon the different divisions of the enterocoele, and incloses 

 the middle portion of the alimentary canal.* 



The Ophiuridea. — The brittle Stars, though they re- 

 semble the ordinary Star-fishes in form, differ essentially from 

 them, not only in the structure of their skeleton, but in the 

 characters of the Echinopaedium. The ambulacra are con- 

 fined to the oral aspect of the body, so that, as in the As- 

 teridea, the ambulacral and oral, the autambulacral and the 

 aboral surfaces, respectively coincide. The mouth is situated 

 in the centre of the oral face, but no grooves radiate from it 

 along the ambulacra, which are covered by a series of plates 

 of the skeleton. The alimentary canal is a simple gastric sac 

 without caeca, and has no intestine or anus. In contradistinc- 

 tion from the Star-fishes, the prolongations of the peritoneal 

 cavity into the rays are very narrow. 



The typical Ophiuridea possess a very complete calcareous 

 skeleton, which, on the body, and on the exterior of the rays, 

 has the form of plates. On the body, the disposition of these 

 varies much ; but five of them, which are situated inter- 

 radiallv in the neis^hborhood of the mouth, are often laro^er 

 than the others, and are termed scuta huccalia. 



Each ray contains an internal solid axis, composed of a 

 single series of quadrate axial ossicles (Fig. 139, C, a), each 

 consisting of two lateral halves united by a longitudinal 

 suture, and articulated together by tenon and mortice joints 

 upon their terminal surfaces. Each of these ossicles (which 

 are sometimes termed vertebral) is surrounded by four plates 

 — one median and antambulacral (Fig. 139, C, h), two lateral 

 (Fig. 139, B, o), and one median and superambulacral (Fig. 

 139, A, d). The lateral plates may meet in the middle line 

 on both the ambulacr.d and the antambulacral faces. Be- 



^ Greef (Z. c.) has worked out the development of Uraster ( Aster acantJ/ion) 

 ruhens^ the larval form of which resembles the Bipinnaria and Brachiolaria 

 of Helsinc^fors, described by Miiller. Parthenogenesis appears to occur in 

 this Star-fish. 



