ECHINODERMA. ECHINOIDEA 135 



of England are Hemiaster and Schizaster. In the Eocene of 

 the South of Europe, India, etc., echinoids are numerous ; 

 the regular forms are less important than in earlier forma- 

 tions, but the Atelostomata,in this and subsequent deposits, 

 become increasingly abundant. The Pliocene echinoids 

 found in East Anglia show considerable affinity to those 

 now living in the West Indian seas ; the principal genera 

 represented are Echinus, Echinocyamus, Spatangus, and 

 Temnechinus. 



CLASS IV. HOLOTHUROIDEA 



This Class includes the sea-cucumbers. They possess 

 an elongated and usually cylindrical body with the mouth 

 at one end and the anus at the other ; around the mouth 

 is a circle of tentacles, which are really modified tube-feet. 

 From the water- vascular ring five radial vessels are given 

 off, and branches also go to the tentacles. In Synapta and 

 its allies tube-feet (with the exception of the tentacles), as 

 well as radial vessels, are absent. The stone-canal in almost 

 all cases opens into the body- 

 cavity. The integument is 

 leathery, and the skeleton is 

 very poorly developed, con- 

 sisting of minute isolated 



„ . , , Fig. 54. A, B, anchor and plate of 



pieces of various shapes, such Synapta tenera, Recent. C, wheel 



as spicules, anchors, and ot Chirodota coiwexa iiom the 

 r Inferior Oolite. Enlarged. 



wheels (fig. 54). 



At the present day the Holothurians are widely dis- 

 tributed, but owing to the nature of their hard parts they 

 are rarely found fossil. The earliest known forms occur in 

 the Carboniferous rocks of Scotland, and a few specimens 

 have been recorded from Jurassic and later formations. 



