568 THE ANIMAL KINGDOM. 



These ossicles take up the greater part of the cavity of each arm. Each 

 one is composed of a right and left half immoveably united by suture. 

 In certain deep-sea Ophiurida, however, each half retains the embryonic 

 character of a curved bar united to its fellow at either end. The ossicles 

 are connected by well-developed articulations, and there is a series 

 of intervertebral, dorsal and ventral, muscles longitudinally disposed by 

 means of which the arms are actively moved, principally in a horizontal 

 plane. The halves of the two first pairs of ossicles are separated. The 

 right half of the first ossicle in one arm approaches the left half of the 

 corresponding ossicle in the adjoining arm to form a paired interradial 

 peristomial plate which is internal, and placed dorsally to the triangular 

 oral angle pieces. Each angle piece projects interradially, and the mouth 

 has consequently a stellate figure. The piece consists of a right and left 

 half united moveably by a joint. And each half has its apex formed by the 

 first adambulacral which is borne upon a support, the second ambulacral of 

 its own side. This second ambulacral is covered externally by the second 

 adambulacral or lateral buccal shield which lies at the side of the buccal 

 shield or oral plate. The ventral margin of each angle piece supports 

 spines, the buccal papillae at the sides, the dental papillae at the apex 

 of the triangle. The vertical oral edges of the conjoined first adambulacrals 

 support a row of sj^ines, the palae angulares with basal pieces. The 

 latter generally fuse to form a bar, the torus angularis. The second 

 ambulacral is pierced by the two first tube-feet or b^lccal feet which pro- 

 trude into the cavity of the mouth. An aperture limited by the first and 

 second adambulacrals and the first superambulacral shield gives exit to the 

 third tube-foot. 



The nerve ring lies on the inner or oral surface of the peristomial plates. 

 The five radial nerves immediately underlie the superambulacral shields. 

 Branches pass outwards on the tube-feet in which there is a sub-ectodermic 

 nerve layer ; and such a layer may perhaps exist beneath the ectoderm of 

 the body-surface. Two genital nerves arise distally to the branches for 

 the buccal feet. The blood-vascular ring and radial vessels lie beneath 

 the corresponding nerve structures. There is an aboral ring or plexus 

 which alternately ascends dorsally on the arms, and descends ventrally in 

 the interradii. The rings and vessels are inclosed between two perihaemal 

 spaces ; a perihaemal space also incloses the stone-canal, and the plexiform 

 organ which connects the two rings in the madreporic interradius. The 

 water-vascular ring has a ciliated stone-canal ending with an ampulla 

 on one of the buccal shields which is pierced by a pore. Some species 

 of Astrophyton have five madreporites (pore plates), one in each interradius. 

 Trichaster elegans is remarkable for possessing five simple pores instead 

 of pore-plates. The stone-canal in these cases is similarly multiplied. 

 The ring sometimes possesses four Polian vesicles, one in each inter- 



