392 ECHINOZOA. 



irregular calcareous plates, or studded by calcareous spines. No dental 

 apparatus is present. The mouth is inferior, and central in position ; 

 the anus either absent or dorsal. The ambulacral tube-feet are pro- 

 truded from grooves on the under surface of the rays. 



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 

 Asterias and Crib?-ellce, and the extinct 

 Palceasters (fig. 270), 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 or ten up to thirty or more. 

 In the Cushion - stars (Goniaster and 



big. 270. — Palceaster JSiiagarensis, . . . v 



Hall. Ordovkian. Gomodiscus), again, the body is pen- 



tagonal, disc-shaped, and flattened on 

 the two sides, and the arms can only be recognised by the ambu- 

 lacral grooves on the lower surface (fig. 271). 



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

 the disc, is the minute 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 " madreporite " or " madre- 

 poriform 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 five pairs of so-called " oral plates " (fig. 271, 0), 

 which must not be confounded with plates similarly named in the 

 Crinoids ; but there are no teeth. Deep furrows, known as the 

 " ambulacral grooves," radiate from the mouth, one along the under 

 surface of each of the arms, gradually narrowing as the extremity of 

 the latter is approached. The roof of each groove is constituted by 

 a double row of minute calcareous pieces — the " ambulacral ossicles " 



