370 ECHINOZOA. 



ball or hemisphere (the " mamelon ") supported upon a conical pro- 

 cess ("the boss") which arises from the plate. The ball of the 

 tubercle may or may not be perforated for the insertion of a liga- 

 ment which is attached to the articular surface of the spine. In 

 many cases (as in fig. 241) the base of the tubercle is surrounded by 

 a round or oval, smooth and excavated space which is termed the 

 "areola" or " scrobicule." 



The spines are movable appendages which are jointed to the tuber- 

 cles by a sort of "ball-and-socket" or "universal" 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 exceeding the diameter of the test. 

 Sometimes they are slender, tapering, and truly spine- 

 like; at other times they are thickened, ovate, or 

 almost globular (fig. 242). The spine fits on to 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 

 Fig. 242.— Spine of the ligament before spoken of. Above the acetab- 

 °diferL ^jurafsic?" ulum or socket of the spine there is a prominent 

 ridge or ring, more or less " milled," for the attach- 

 ment of the muscular fibres which move the spine. 



At the summit of the corona in all Echinoids is found the single 

 or double row of plates which constitutes the " apical disc " or dorso- 

 central system, corresponding with the " calyx " of the Crinoids. In 

 the most typical forms (" Regular " Echinoids) the plates of the 

 apical disc surround the membrane ("periproct ") in which the anus 

 is situated (fig. 243, a); but in the so-called "Irregular" Echinoids 

 (fig. 243, b) the apical disc simply occupies the summit of the 

 test, and the anus is excentric, and is entirely removed from the 

 disc. In its most ordinary condition (as in the genus Echinus, fig. 

 237), the apical disc is composed of ten plates arranged in two 

 alternating rows of five plates each. The uppermost row consists 

 of a cycle of five large plates of a pentagonal form, which are perfor- 

 ated each by the duct of a testis or ovary, and are therefore known 

 as the "genital plates." (In the Palaeozoic Echinoids from three 

 to five pores pierce each genital plate.) 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 " madreporite " or " madreporiform tubercle" (fig. 243, ma). 

 The madreporite varies much in size, and sometimes forms almost 

 the whole of the apical disc. The genital plates occupy the summits 

 of the interambulacral areas, and are therefore interradial in position. 

 Wedged in between the genital plates, and occupying the summit 



