188 PHYLUM ECHINODERMATA. 



group, and in some cases it appears that a single arm can re- 

 produce the whole animal. This appears to be the explanation 

 of the so-called comet-forms, which consist of a large arm carry- 

 ing a small disc with four small arms. Fission of the body 

 through the centre of the disc into two parts sometimes occurs. 

 The power of regeneration possessed by a wounded surface 

 sometimes leads to the production of curious forms, e.g. in 

 Linckia multifora the wounded surface of an arm has been 

 described as forming a new disc with four arms. 



The development (p. 133) is rarely direct, and the young 

 usually pass through the free-swimming larval stage called 

 bipinnaria. Including brachiolaria larvae about twenty bipin- 

 naria larvae are known. Most of these have not been related 

 to their adults. The larva of Asterina gibbosa may be regarded 

 as a much modified bipinnaria ; it has the power of swimming 

 feebly with the cilia of its larval organ. 



Bipinnaria asterigera is the larva of Luidia sarsii ; it is the 

 largest bipinnaria known (1-licm.). B. metschnikoffi and 

 mulleri probably belong to species of Astropecten. B. russoi and 

 buryi have been assigned to Asterias glacialis. 



The Asteroidea are found fossil from the Cambrian onwards, 

 but the known fossil forms are not nearly so numerous as in the 

 case of Oinoids and Echinoids. This is doubtless due to the 

 fact that their tissues do not lend themselves so readily to pre- 

 servation as do those of the above-named classes. The early 

 forms do not differ essentially from those now living. The 

 class is divided into two orders the Encrinasteriae, in which 

 the ambulacral plates alternate on the two sides of the arm and 

 the madreporite is on the lower surface ; and the Euasteriae, in 

 which the ambulacral plates are opposite one another and the 

 madreporite is on the dorsal surface. The Encrinasteriae, are 

 exclusively Palaeozoic, while the Euasteriae include all the living 

 forms and make their first appearance in the Silurian. 



Order I. ENCRINASTERIAE. 

 With characters as above. 



Aspidosoma Goldf. lower Devonian ; Palaeaster Hall (Archasterias J. 

 Mull), Silurian, Devonian and Carboniferous; Urasterella M'Coy (Stenas- 

 ter Billings), lower Silurian ; Palasterina McCoy ; Palaeodiscus Salter ; 

 Palaeocoma Salter, upper Silurian ; Salteraster, etc. 



