292 ECHINODEBMATA. 



(Stelleridea) movably articulated by teeth, which tit into one another 

 at the bottom of the anibulacral groove ; the latter only (Stelleridea) 

 possess transverse muscles 011 the anibulacral ossicles, and are able 

 to bend their arms together towards the ventral surface. The 

 Ophiuridea are provided with longitudinal muscles only, by means 

 of which they are able to bend their arms to the right and left in 

 a horizontal plane with a serpentine movement. 



The mouth is always placed in the centre of the ventral surface in 

 a pentagonal or star-shaped depression, the edges of which are 

 usually beset with stiff papillae. The inter-radial angles are marked 

 by the junction of two adambulacral plates, and frequently function 

 as organs of mastication. The anus may be wanting ; when present, 

 it invariably lies at the apical pole. The madreporic plate, of 

 which there may be one or more, is situated inter-radially and 

 dorsally (Stelleridea), or on the inner surface of one of the buccal 

 plates (Ophiuridea), on the exterior of which a pore may be present. 

 Development in certain cases takes place without the interposition 

 of a bilateral larval phase with bands of cilia. When such larvae 

 are developed, they have the form of a Pluteus (Ophiurid) or Bipin- 

 naria and BracJtiolaria (Stellerid). 



The great power of regeneration possessed by Starfishes is not 

 confined to the reproduction of lost arms, but may lead to the new 

 formation of portions of the disc, or even of the entire disc from 

 a single separated arm. This process thus amounts to a species of 

 asexual reproduction by fission, and has been especially observed in 

 forms with six (Ophiactis) or more than six (Linckia) arms. 



Fossil star-fishes are found as far back as the lower Silurian strata 

 (Palceaster), where intermediate forms between Stelleridea and 

 Ophite-idea (Protaster) make their appearance. 



Sub-Class 1. STELLERIDBA (Aster idea) STARFISHES. 



Asteroidea ^vhose arms are, prolongations of the disc, and contain the 

 hepatic appendages of the alimentary canal, and also the, generative 

 organs. They possess a deep, uncovered anibulacral groove running 

 along the ventral surface, in which groove the ambulacral feet are 

 disposed in roivs. 



The Stelleridea usually possess broad arms, and are characterised 

 by the fact that the ambulacral ossicles of the two sides are connected 

 by transverse muscles and are movable upon one another. The 

 anus lies at the aboral pole, but may be wanting in certain genera 

 (Astropecten). The madreporic plate and the genital pores are 



