ASTEROIDEA. 1 1 1 



inferior in position, and is toothless. An anus is usually present, 

 but may be absent. 



The two most striking features which distinguish the Star- 

 fishes from the Sea-urchins are the star-like figure of the 

 former, and the fact that the body is not enclosed in an 

 immovable calcareous box or " test," as it is in the latter. 

 The integument of the Asteroidea is, however, so richly pro- 

 vided with calcareous matter, that though more or less soft 

 and flexible during life, it is quite capable of being preserved 

 in a fossil condition. It is, of course, wholly with the cal- 

 careous secretions of the animal that the palaeontologist has to 

 deal ; and we may, therefore, dispense with any further account 

 of the soft parts, beyond what is contained in the above defini- 

 tion. 



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 Cribella, 

 and the extinct Palceasters (fig. 65), 

 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 num- 

 ber, from eight or ten up to thirty 

 or more. In the Cushion-stars 

 (Goniaster and Goniodiscus), again, 

 the body is pentagonal, disc-shaped, 

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

 recognised by the ambulacral grooves on the lower surface 

 (fig. 66). 



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 



