426 ZOOLOGY 



SECT. 



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 developing a new 

 disc and a completed series of arms. In some Starfishes (Ophiuroids 

 and Asteroids) a process of separation of the arms and their develop- 

 ment into complete individuals frequently occurs altogether inde- 

 pendently 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 Comatulse 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 Meso- 

 zoic Period that undoubted traces of their existence are found. 



Affinities. The presence of radial symmetry was formerly 

 regarded as involving a near relationship with the Ccelenterata, 

 which were grouped with the Echinodermata under the comprehen- 

 sive class-designation of Radiata (see section on the History of 

 Zoology). But, leaving out of account the presence of a bilateral 

 symmetry underlying and partly concealed by the radial, we are led 

 by a study of the anatomy of the various systems of organs to the 

 conclusion that the Echinoderms are in no way closely or directly 

 related to the Ccelenterates. One very great and very important 



