CLASSIFICATION. 133 



creeping creatures, moving by means of their tube-feet or by 

 their arms (Comatulids, Ophiuroids). Most Crinoids how- 

 ever are attached by a stalk which is a prolongation of the 

 aboral surface of the body, but they may become detached and 

 acquire a new attachment (Pentacrinus). 



From a psychical or nervous point of view, the Echinoderms 

 are extremely low in the scale of life, but judging them from the 

 complexity of their organization apart from the nervous system, 

 they approximate to the so-called higher animals. Geologically 

 they are of great interest, the structure of the body wall lending 

 itself very readily to their preservation as fossils. They make 

 their appearance in the Cambrian and in the Silurian with a 

 range of structure not unlike that which characterizes living 

 forms. 



The power of regenerating lost parts is considerable in almost 

 all Echinoderms, and many of them, especially those with long 

 arms, possess the power of autotomy. It is least developed 

 in Echinoids, but in Asteroids and Ophiuroids it is in some cases 

 so extensive that not only may arms and portions of the disc 

 be regenerated, but the whole body may be reformed from a 

 single arm. In Crinoids and Holothurians the viscera even are 

 capable of regeneration. 



Asexual reproduction is found in many Asteroids, Ophiuroids 

 and Holothurians. It takes the form of fission into two equal 

 halves, in the two former classes the plane of fission passing 

 through the disc, in the latter transversely to the long axis of the 

 body. 



In this work the Echinodermata are divided into seven 

 classes : Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea, 

 Crinoidea, Cystidea, Blastoidea. The mutual relationships of 

 these are discussed under the accounts of each class. The group 

 Pelmatozoa is considered and the reasons for not adopting it 

 are stated on p. 303 et seq. 



Development.* In the great majority of Echinoderms the 



* For a more complete account of the developmant of the Echinoder- 

 mata, the readsr is referred to the following works, where the older litera- 

 ture will be found : F. M. Balfour, A Treatise on Comparative Embryology, 

 vol. 1, London, 1885; E. Korschelt and K. Heider, Textbook of the Em- 

 bryology of Invertebrates, English Translation, London, 1895. A. Lang, 

 Text-Book of Comparative Anatomy, English Translation, vol. 2, 

 London, 1896. Of recent workers the following may be cited : E. W. 

 MacBride, The development of Asterina gibbosa, Q.J.M.S., 38, 1896, p. 



