238 THE STELLEROIDEA 



groove in the Asteroids and its absence in Ophiuroids ; and the 

 restriction of the digestive and generative organs to the disc, and 

 consequent sharp distinction between body and arms, in the latter. 

 The first character is unreliable, as in the living Ophiotcresis there 

 are no ventral plates, and a shallow ambulacral furrow is accord- 

 ingly present. One order of fossil Ophiuroids the Lysophiurae 

 has the same feature more strongly marked. The differentia- 

 tion of the body into disc and arms happens in most Ophiuroids, 

 but also in some Asteroids, e.g. Freyellci. The restriction of the 

 digestive organs to the disc appears to offer a more reliable 

 character ; but in Astrophiura the arms are sharply marked off 

 from the disc and contain no digestive caeca, while the ambulacral 

 ossicles are asteroid. Similarly the digestive and genital systems 

 of the Asteroids, Colpaster, and some species of Freyella must be 

 limited either to the well-marked disc, or at most to the bases of 

 the arms ; while the arm structure is practically identical with 

 that of some Palaeozoic Ophiuroids. It must be remembered, 

 moreover, that the digestive sac of Ophiuroids is marked by a 

 series of radial bulgings, which may be homologous with the 

 radial caeca of Asteroids. The position of the madreporite is relied 

 on by Perrier, but it will not serve ; in the Asteroids, Asterina, 

 and Palasteriscus it is ventral, as in most Ophiuroids. 



Not only is there no character which serves to separate the 

 Ophiuroids and Asteroids, but the whole structure of the body is 

 on the same plan in both groups. It consists in both of a central 

 disc and a series of (usually five) radial rays. The skeleton in 

 each ray consists essentially of two series of plates the ambu- 

 lacral and adambulacral. The former lie internal to the radial 

 water-vascular vessel, and the furrow which this occupies is later- 

 ally protected by the adambulacral plates. Additional elements 

 may occur, but are not found in all members of either division. 

 The mouth armature consists of a ring round the mouth, formed 

 by the union of one or more pairs of ambulacral and adambulacral 

 plates for each arm, and bearing spines modified to serve as teeth. 

 The body is protected by accessory plates or granules in the integu- 

 ment ; these plates may be protected by spines arid pedicellariae. 



The alimentary system consists of a central digestive sac, 

 opening by a mouth at the centre of the ventral surface ; the size 

 of the digestive sac is increased by radial bulgings, of which there 

 are as many pairs as the Stelleroid has arms ; these bulgings may be 

 short and limited to the disc or base of the arms, or extend up 

 the arms. There may or may not be an anus. 



The water-vascular system consists of a ring round the 

 oesophagus ; a radial vessel runs up each arm from the ring, which 

 also bears a series of Polian vesicles, or sac-like diverticula. The 

 radial vessels give off a pair of branches in each segment ; each 



