248 MINERALS AND GEOLOGY 



VIII. 

 HOLOTHUROIDEA. 



This class comprises a small number of elongated, more or less 

 vermiform types, protected by a thick, coriaceous integument, 

 strengthened by calcareous wheel-like, anchor-shaped, and other 

 spicules. The mouth is situated at one extremity of the body, and 

 is surrounded by a circle of quinary, usually branching tentacles. 

 The anal orifice in all the typical forms is at the opposite end of the 

 body ; but in the genus Rhopalodina (Gray) it is at the same extrem- 

 ity as the mouth. Wheel-shaped spicules, thought to have belonged 

 to holothurans, have been found occasionally in carboniferous and 

 higher strata, but apart from this, fossil forms are of very doubtful 

 occurrence. 



SUB-KINGDOM V. 

 VERMES. 



This division, comprising the various parasitic and other worms, 

 with some related memberless types, is of comparatively little palae- 

 ontological interest. It may be subdivided into the six following 

 classes: Turbellaria, Platyelmintha, Nematelmintha, Rotifera, Geph- 

 yrea, Chcetognatha, and Annelida. 



The Turbellaria are non-parasitic, mostly marine or fresh water 

 forms, more or less depressed in shape, but of great length in certain 

 genera (Linus, Borlasia). Fossil forms are unknown. 



The Platyelmintha comprise the so called " flat worms " most of 

 which are internal parasites. Fossil forms unknown. 



The Nematelmintha comprise the so called " round worms " most of 

 which are also permanently, or for a time, parasitic. Of late years 

 some supposed representatives of this class have been discovered in 

 amber aud brown coal, both of Cainozoic age ; but otherwise there 

 are no known fossil representatives. 



The Rotifera are very minute aquatic forms, with one or more 

 ciliated discs at the front end of the body. They are commonly 

 known as " wheel animalculse." There are no recognized fossil 

 forms. 



