HOLOTHUROIDEA. 235 



tinguished from that of the body cavity ; it contains corpuscles, some of 

 which are pigmented. 



On the area round about the mouth there are ten hollow 

 outgrowths, which resemble the skin-gills of star-fishes. As 

 already mentioned, the pigmented cells of the body cavity 

 fluid seem able to absorb oxygen. The water vascular 

 system plays here a very important part in respiration. 

 Waste products seem simply to accumulate in the tissues, 

 but Hartog maintains that the water vascular system helps 

 in excretion. 



The sexes are separate, and like one another. Five 

 branched yellow-brown ovaries or rose-white testes lie 

 interradially under the apex of the shell, and open by 

 separate ducts on the five genital plates. In spring the 

 apical disc may be seen covered with orange ova or milky- 

 white spermatozoa. 



The eggs are fertilised externally by sperms wafted from 

 adjacent sea-urchins, and the free-swimming larva is called 

 a Pluteus. 



Classification. — 



1. PalEeoechinoidea. Extinct forms, apparently with a plastic test 



of overlapping and variable plates. They appear in Lower 

 Silurian rocks. 



2. Desmosticha. Regular and symmetrical sea-urchins like Echinus. 



In Cidaris, there are no external gills. A species of Diadema 

 has been described as covered with compound eyes. In 

 Cyatiosoma tirens the spines icontain a poison apparatus. 

 Echinothuridae have flexible tests. 



3. Clypeastroidea. Shield-shaped, and often flat. The food canal 



ends outside the apical disc on the posterior inter-radius. 

 e.g. Clypeaster. 



4. Petalosticha. Heart-shaped. The mouth is ex-centric, the food 



canal ends away from the apical disc. There are no masticat- 

 ing organs. On the dorsal surface the ambulacral areas dilate 

 , from the apex outwards, and contract again towards the margin 

 in the form of "petals." The anterior area is often different 

 from the other four. 



e.g. Spatangus. Some, e.g. Hemiaster, carry their young 

 among their spines. 



Class Holothuroidea. Sea-Cucumbers. 



The Holothurians do not at first sight suggest the other 

 Echinoderms, for they are like plump worms, and the 



