ANNULOIDA I ECHINODERMATA. 



159 



contact with one another. The plates of the test are studded 

 with large tubercles, which are more numerous on the inter- 

 ambulacral areas than on the ambulacral, and are wanting on 

 all the plates which do not belong to either area. These 

 tubercles carry spines (fig. 56), used defensively and in loco- 

 motion, which are articulated to their apices by means of a sort 

 of " universal " or " ball-and-socket " joint. Occasionally a 

 small ligamentous band passes between the head of the tubercle 

 and the centre of the concave articular surface of the spine, 

 thus closely resembling the " round ligament" of the hip-joint 

 of man. Besides the main rows of plates just described, form- 

 ing the so-called " corona," other calcareous pieces go to make 

 up the test of an Echinus. The mouth is surrounded by a 

 coriaceous peristomial membrane, which contains a series of 



Fig. 56. Cidaris papillata. (After Gosse.) 



small calcareous pieces, known as the " oral plates ; " whilst a 

 corresponding series of " anal plates " is found in the mem- 

 brane surrounding the opposite termination of the alimentary 

 canal. Surrounding the aperture of the anus at the summit of 

 the test is the " apical disc," composed of the so-called genital 

 and ocular plates (fig. 55, 3). The "genital plates" are five 

 large plates of a pentagonal form, each of which is perforated 

 by the duct of an ovary or testis. One of the genital plates is 

 larger than the others, and supports a spongy tubercle, per- 

 forated by many minute apertures, like the rose of a watering- 

 pot, and termed the " madreporiform tubercle." The genital 

 plates occupy the summits of the interambulacral areas. 

 Wedged in between the genital plates, and occupying the 

 summits of the ambulacral areas, are five smaller, heart-shaped, 



