ix PHYLUM ECHINODERMATA 435 



the co-ordination of movement is very much more complete in the 

 E&chinodermata than in the groups already dealt with. 



A remarkable characteristic of the Echinoderms is the faculty 

 of si-//-))intilaHon which many of them possess, together with 

 the capacity for replacing parts lost in this way or by acci- 

 dental injury. This is most marked in many Ophiuroids, some 

 Asteroids, and some Holothurians, and does not occur at all among 

 the Echinoids. Many Brittle-stars and some Starfishes, when 

 removed from the water, or when molested in any way, break off 

 portions of their arms piece by piece until, it may be, the whole 

 of them are thrown off to the very bases, leaving the central disc 

 entirely bereft of arms. A central disc thus partly or completely 

 deprived of its arms is capable in many cases of developing a new 

 set ; and a separated arm is capable in some instances of develop- 

 ing a new disc and a completed series of arms. In some Star- 

 fishes (Ophiuroids and Asteroids) a process of separation of the 

 arms and their development into complete individuals frequently 

 occurs altogether independently of injury, and seems to be a 

 regular mode of reproduction in these exceptional cases. Many 

 Crinoids, also, readily part with their arms when touched and are 

 able to renew them again ; and some, at least, are capable 

 of renewing the visceral sac of the central disc when it has become 

 accidentally removed. 



In the case of many Holothurians it is the internal organs, or 

 rather portions of them, that are capable of being thrown off and 

 replaced — the oesophagus, or the cloaca with the Cuvierian organs, 

 or the entire -alimentary canal, being ejected from the body by 

 strong contractions of the muscular fibres of the body-wall, 

 and in some instances, at least, afterwards becoming completely 

 renewed. 



Four out of the nine classes of the phylum Echinodermata — 

 the Cystoidea, Blastoidea, Edriasteroidea, and Carpoidea — are 

 represented only by fossil forms; and these are found only 

 in rocks of the older (Palaeozoic) formations, no representatives 

 having survived to more recent times. Of the five classes that 

 have living members, one, the Crinoidea, was very much more 

 abundantly represented in the older geological periods than 

 it is at the present day, the remains of stalked Crinoids 

 forming great beds of limestone of Silurian to Carboniferous 

 age : the free Comatulae only appeared at a much later period. 

 The other classes, or at least the Echinoidea, Asteroidea, 

 and Ophiuroidea, were represented at a very early period by 

 forms not very widely different from those now living ; but the 

 earliest Echinoids were peculiar in having the number of rows 

 of plates variable, and in the plates overlapping one another. 

 The Holothuroidea, owing to their comparatively soft integument, 

 were less fitted to leave any remains in the form of fossils, and it 



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