ASTEROIDEA. 245 



In their form the Star-fishes differ considerably, though in 

 most the figure is markedly stellate. The animal consists of 

 a central body or " disc," which gives off radiating processes 

 or " arms," but the size of the disc is very different in different 

 species, and the arms vary greatly in length and in number. 

 In many living and extinct forms the " disc " is inconspicuous, 

 and appears to be formed simply by the junction of the bases 

 of the arms, which in this case are normally five in number. 

 The living Urasters and Cribellce, 

 and the extinct Palceasters (fig. 

 139), may be taken as examples 

 of this state of parts. In other 

 forms, as in the Sun-stars (Solaster) 

 and the extinct Lepidasters and 

 Plumasters, the disc is very broad, 

 exceeding or equalling the length 

 of the arms in its diameter ; whilst 

 the rays vary in number, from eight 



J J Fig. 139. Palceaster Niagarensds, 



or ten up to thirty or more. In Hail. Lower Silurian, 



the Cushion - stars (G-oniaster and 



Goniodiscus), again, the body is pentagonal, disc-shaped, and 

 flattened on the two sides, and the arms can only be recog- 

 nised by the ambulacral grooves on the lower surface (fig. 

 140). 



On the upper surface of the body, placed nearly in the 

 centre of the disc, is the aperture of the anus, when this is 

 present ; but the genera Astropecten, Ctenodiscus, and Luidia 

 are destitute of a vent. Also on the upper surface is the 

 " madreporiform tubercle," in the form of a spongy or striated 

 disc placed at the angle of junction of two rays. It has the 

 same function as in the Echinoids, serving to protect the 

 entrance to the water-vascular system. Ordinarily there is 

 a single madreporiform tubercle, but in some genera there 

 are two, three, or more tubercles ; and there seems in some 

 cases to be a correspondence between the number of the 

 arms and the number of madreporic plates. 



Placed in the centre of the lower surface is the mouth, at 

 the angles of which are the so-called "oral plates" (fig. 140). 

 Eadiating from the mouth are a series of furrows, varying in 



