ANNULOIDA : ECHINODERMATA. 



125 



responding series of 'anal plates' is found in the membrane 

 surrounding the opposite termination of the alimentary canal. 

 In addition to these minute ossicula, at the summit of the test 

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

 ocular plates {fig. 32, 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 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.' 



Pig. 32. Morphology of Echinoidea. 1. Portion of the test of Galerites 

 hemisphericus enlarged, showing the inter-ambulacral area (a), and the 

 ambulacral areas (b). 2. Galerites hemisphericus viewed from above, a. 

 Inter-ambulacra ; 6. Ambulacra. 3. Genital and ocular disc of HemiadaHs 

 intermedia, enlarged, c. Ocular plate ; d. Genital plate ; e. Anal aperture ; 

 /. Madreporiform tubercle. 4. Spine of the same. (After Forbes.) The 

 tubercles are mostly omitted on figs. 2 and 3 for the sake of clearness. 



Besides the spines, which are sometimes of a very great 

 length, the test often bears curious little appendages, called 

 'pedicellarise,' and often supposed to be parasitic. Each of 

 these consists of a stem, bearing two or three blades or claws, 

 which snap together and close upon foreign objects, like the 

 beak of a bird. Their action appears to be independent of 

 the will of the animal, and their true function is not known. 



Locomotion in the Echinoidea is effected by means of a 



