IT)4 A Guide to the Zoological Collections m thi 



collection by fine specimens of Pentacrinus, Metacrinus^ 

 Actinometra and Antedon, — all from thelndian Seas. 



The extinct species are represented by fossil speci- 

 mens of Pentacrinus from the Permian, and of Penta- 

 tremites from the Carboniferous. 



4. ECHINODERMATA ECHINOIDEA (SEA-URCHINS, SEA 



EGGS). 



[Eastern giEaU-£a0C0 28-34]. 



In the Echinoidea the body is spherical, or heart- 

 shaped, or disk-shaped, and is always enclosed in a shell 

 or test formed of calcareous plates which in all living 

 forms, except Phormosoma and Asthenosoma, are immov- 

 ably sutured together. These calcareous plates are always 

 more or less densely covered with movable spines — there 

 usually being a close covering of small spinelets, with very 

 much larger spikes interspersed. Three-pronged pedicel- 

 lariae are very commonly found among the spinelets, these 

 in Cidaris, for example, being large enough to be easily 

 distinguished by the naked eye. 



The external characters of a typical Regular Echinoid 

 are well shown in the specimen of heterocentrotus trigon' 

 arius (Case 28), which has been dried, denuded of all its 

 spines, and coloured. 



Here, looking from above, we find the plates of which 

 the test is composed to fall into twenty meridional rows 

 arranged in ten pairs. Five of these paired rows of 

 plates are narrow, and are closely perforated with numer- 

 ous pairs of pores. These pores are for the passage of 

 the ambulacral tube-feet. The perforated plates are there- 

 fore known as ambulacral plates, and the five rows of 

 them are collectively known as the " radii ". They are 

 coloured crimson in the specimens. 



Alternating with the radii are five also double rows of 

 very much broader plates which are not perforated : these 

 are known collectively as the " interradii ", and are coloured 

 blue in the specimens. 



