GENERAL CHARACTERS OF THE ECHINODERMATA. 365 



mentj and usually creep about by the aid of their tube-feet, with 

 the oral surface of the body turned downwards. They are there- 

 fore grouped together in a common division under the name of 

 Echinozoa. 



As regards their distribution^ all the recent Echinoderms are mar- 

 ine, and from their habit of life and their possession of a calcareous 

 exoskeleton, the members of this sub-kingdom are largely represented 

 as fossils, ranging from the Upper Cambrian period onwards. The 

 classes of the Cystoidea and Blastoidea are not only extinct, but are 

 exclusively Palaeozoic ; while in the Cri?widea we have a group 

 which has passed its prime, and appears to be verging on extinction. 

 On the other hand, the classes Echinoidea, Asteroidea, Ophiuroidea, 

 and Holothuroidea appear to have attained their maximum of de- 

 velopment at the present day. The Asteroidea and Ophiuroidea 

 commence in the Ordovician period. The Echinoids commence in 

 the Ordovician, but reach no marked development till we enter 

 upon Mesozoic deposits. Lastly, the Holothurians, as might be 

 expected from the soft nature of their integuments, are hardly known 

 as fossils, though they seem to have existed at any rate as early as 

 the Carboniferous period. 



