ECHINODEEMAT A . 



225 



circulation of the blood, and the highest order have a com- 

 plicated branched tree-like organ for respiration. In other 

 orders the water is admitted freely into the interior of the 

 shell, and aerates the blood as it circulates through the body. 

 The ova are developed in a long complicated ovary. 



The class is divided into four orders HOLOTHUEIDA, 



ECHINIDA, ASTEEIDA, CEINOIDA. 



The HOLOTHUEIDA (sea-cucumbers) have a soft con- 

 tractile body enclosed in a muscular and coriaceous integu- 

 ment with tentacula around the mouth, and rows of tubular 

 feet, disposed in longitudinal lines along the sides of the body. 

 None of this order have yet been found in a fossil state. 



ECHINIDA (sea-urchins) have a globular ovoid or de- 

 pressed body without rays. Their shell is composed of a 

 series of polygonal calcareous plates arranged in columns. 

 One series of plates have their surface covered over with 

 tubercles for supporting moveable spines. Another series 

 have perforations for the passage of tubular contractile feet. 



The mouth and the vent have distinct openings which 

 occupy different relative positions on the shell; some- 

 times the vent is situated on the summit sometimes on 

 the posterior border 

 sometimes between the 

 border and the summit 

 sometimes at the under 

 surface of the shell, be- 

 tween the mouth and the 

 border. 



The Echinida is divi- 

 sible into three families 

 the CLTPEASTEOID^, 

 the SPATANGOLDJG, the 



FIG. 157. 1. Nucleolites. 2. Cidaris diadema. 



3. Micraster cor-anguiuum. 



4. Ananchytes ovata. 



Each family attained 

 its greatest development 

 at different periods. 



The CLYPEASTEOID.E, 

 of which Nucleolites is typical, first appeared in the oolitic 

 rocks ; they are found in greatest numbers in the tertiary 

 beds, and in our modern seas.* 



* Pict. Atlas, pi. liv. 



