106 ANNULOIDA. 



to the tubercles by a sort of "ball-and-socket" or "uni- 

 versal" joint. They are used defensively and in locomotion, 

 and vary much in length and shape. Sometimes they are very 

 minute ; at other times they attain a length considerably ex- 

 ceeding the diameter of the test Sometimes they are slender, 



Fig. 60. Hemicidaris crenularis, showing tubercles, the larger of which are 

 perforated, and are surrounded by an areola. Oolite. 



tapering, and truly spine-like ; at other times they are thick- 

 ened, ovate, or almost globular (fig. 61). The spine fits on 

 the rounded head of the tubercle by a concave 

 articular surface ("acetabulum"), and there may 

 or may not be a pit at the bottom of this, for 

 the attachment of the ligament before spoken 

 of. Above the acetabulum or socket of the 

 spine there is a prominent ridge or ring, more or 

 less " milled," for the attachment of the muscular 

 fibres which move the spine. 



The " apical disc " or " genital disc " occupies 

 the summit of the test, and is generally composed 

 Fig. .61. spine of ten plates (fig. 57, 3). Five of these plates 

 We slan are of comparatively large size, and are termed 

 the "genital plates," each being perforated by 

 the duct of an ovary or testis. Each genital plate occupies 

 the summit of one of the interambulacral areas. One of the 

 genital plates (the right antero-lateral plate) is larger than the 

 others, and supports a spongy tubercle, perforated with many 

 apertures, like the rose of a watering-pot, and termed the 

 " madreporiform tubercle" (fig. 57). This structure protects 

 the mouth of the canal by which water is admitted from the 

 exterior to the water-vascular system. Wedged in between 

 the genital plates, and occupying the summits of the ambulacral 

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

 of which is perforated for the reception of an " ocellus " or 

 eye, and which are therefore termed the " ocular plates." 



