146 



MANUAL OF ZOOLOGY. 



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 re- 

 sembling the " round ligament" of the hip-joint of man. Be- 

 sides 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 

 surrounding the opposite termination of the alimentary canal. 

 Surrounding the aperture of the anus at the summit of the test 

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



Fig. /^o.— Cldarls fnpLUaia. (After Gosse.) 



ocular plates (fig. 39, 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, perforated by . 

 many minute apertures, like the rose of a watering-pot, and 

 termed thfe " 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, or penta- 

 gonal plates, known as the " ocular plates," each being per- 

 forated by a pore for the reception of an "ocellus" or " eye." 

 Besides the spines, which are sometimes of a very great 

 length, the test often bears curious little appendages, called 

 " pedicellariae," and often supposed to be parasitic. Each of 



