ECHINODERMATA : ECHINOIDEA. 



195 



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 



Fig, 94. Cidaris papillata. (After Gosse.) 



resembling the "round ligament" of the hip-joint of man. 

 Besides the main rows of plates just described, forming 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 small cal- 

 careous pieces, known as the " oral plates ; " whilst a corre- 

 sponding series of "anal plates" is found in the membrane 

 (fig. 93, C, /) 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. 93, C). 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, perforated by many minute apertures, like 

 the rose of a watering-pot, and termed the " madreporiform 

 tubercle" (fig. 93, C, m). In some cases, this tubercle is not 

 connected with one of the genital plates, but is placed in the 

 centre of the apical disc. The genital plates occupy the 

 summits of the inter-ambulacral areas. Wedged in between the 

 genital plates, and occupying the summits of the ambulacral 

 areas, are five smaller, heart - shaped, or pentagonal plates, 

 known as the " ocular plates," each being perforated by a pore 

 for the reception of an " ocellus " or " eye." (The existence 

 of an eye-spot is denied by high authorities.) 



