240 ECHINODEiaiA. 



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. 



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. Paloeo-echinoidea. 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 Ectiiuus. 



e.g., Cidaris, without e.xternal gills. 



Diade?)ia, a species has been described as covered w"ith 



compound eyes. 

 Cyanosoina ureits, the spines contain a poison apparatus. 

 Echinothuridn; 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 to^^"ards the margin 

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

 from the other four. 

 e.g., Spatangiis. 



Heiniaster and some others carry their young among 

 their spines. 



Class HoLOTHUROiDE.\. Sea Cucumbers. 



The Holothurians do not at first sight suggest the other 

 Echinoderms, for they are like plump worms, and the 

 calcareous skeleton is not prominent. But closer examina- 

 tion shows the characteristic pentamerous symmetry, and 

 the occurrence of calcareous plates in the skin. These 

 seem to be absent in the unique pelagic Pelagothuria. 



Holothurians occur in most seas, from slight to very 

 great depths. Their food consists of small animals, and of 



