400 ZOOLOGY SECT. 



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

 Echinodermata than in the groups already dealt with. 



A remarkable characteristic of the Echinoderms is the faculty 

 of self-mutilation 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 

 capable of renewing 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. 



Two out of the seven classes of the plylum Echinodermata 

 the Cystoidea and Blastoidea are represented only by fossil forms ; 

 and these are found only in rocks of the older (Palaeozoic) forma- 

 tions, 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 geo- 

 logical 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 ComatulaB 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 

 is not till we come to the Mesozoic Period that undoubted traces 

 of their existence are found. 



