STRUCTURE OF A SEA-URCHIN. 213 



blood vessels or spaces in the ambulacral areas. The fluid 

 contains corpuscles, some of which have brown pigment. 



Respiratory and Excretory Systems. — On the area round 

 about the mouth there are ten hollow outgrowths, which 

 resemble the skin-gills of starfishes, and have probably a 

 similar function. As already mentioned, the pigmented 

 cells of the body-cavity fluid seem able to absorb oxygen. 

 The observation that currents pass out of the madreporic 

 plate, suggests that the water-vascular system has in part an 

 excretory function. 



Reproductive System. — 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 or basal plates. 

 In spring the apical disc may be seen covered with orange 

 ova or milky-white spermatozoa. At this season in the 

 south of Europe the sea-urchins are brought to market. 



The eggs are fertilised externally by sperms wafted from 

 adjacent sea-urchins, and the free-swimming larva, which we 

 shall afterwards describe, is called a Pluteus. 



Classification of Echinoidea. 



1. Pateo-echinoidea. Extinct forms, apparently with a jDlastic test, 



of over-lapping and variable plates. They appear in Lower 

 Silurian Rocks. 



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



e.g. , Cidaris, without external gills. 



Diadema, a species has been described as covered with 



compound eyes. 

 Cyanosoma iirens, the spines contain a poison apparatus. 

 Echinothuridss,have flexible tests. 



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



ends outside the apical disc on the posterior inter-radius 

 (exocyclic). 

 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., Sfatangus. 



Hemiaster and some others carry their young among 

 their spines. 



