226 



ANNULOIDA. 



ORDER I. ECHINOIDEA. 



The members of this order commonly known as Sea- 

 urchins are characterised by the possession of a more or 

 less globular, heart-shaped, discoidal or depressed body, encased 

 in a " test " or shell, ivhich is composed of numerous calcareous 

 plates, immovably connected together. The intestine is convoluted, 

 and there is a distinct anus. The mouth is usually armed 

 with calcareous teeth, and is always situated on the inferior 

 aspect of the body, but the position of the vent varies. 



As a matter of course, the palseontological student has to 

 deal with nothing except the test of the Echinoids and its 



Fig. 117. Morphology of Echinoidea. A, Young specimen of Strongylocentrotus Drobacli- 

 iensis, viewed from above. B, Small portion of the test of the same, magnified, c, Summit 

 of the test of Echinus sphcera, magnified. D, Clypeaster subdepressus, viewed from above, 

 showing the petaloid ambulacra. E, Spine of Porocidaris purpurata. F, Pedicellaria of Tox- 

 opneustes lividus. a, a, Ambulacral areas ; I, i, Interambulacral areas ; g, Genital plate ; o, 

 Ocular plate ; ra, Madreporiform tubercle ; p, Membrane surrounding the anus. (Figs. A, B, 

 fyid D are after A. Agassiz.) 



appendages, and these must be described in some detail. 

 The " test " of the Echinoidea may be regarded as essentially 

 composed of the so-called " corona " and of the " apical disc/' 



