244 



CHAPTER XIII. 



ASTEROIDEA AND OPHIUROIDEA. 



ORDER II. ASTEROIDEA. 



THE order Asteroidea or Stellerida comprises the ordinary 

 " star-fishes," and is defined by the fact that the body (fig. 139) 

 is star-shaped or pentagonal, and consists of a central " disc" 

 surrounded by five or more lobes or " arms" The arms are 

 truly prolongations of the body, are hollow, and contain pro- 

 longations of the stomach in their interior. The arms are, 

 further, grooved on their under surface for the reception of the 

 ambulacral or water-vascular vessels. From these grooves the 

 tube-feet are protruded in two or four rows. The integument 

 (perisome} is leathery, but is more or less calcified ly the de- 

 velopment in it of plates, granules, and spines of carbonate of 

 lime. The mouth is 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 im- 

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

 integument of the Asteroidea is, however, so richly provided 

 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 definition. 



