258 ECHINODERMS OF THE BRITISH ISLES 



deepened, circular area, the " areole'' (Fig. 142), the place of 

 attachment of the spine muscles. 



The S2)i)ies are usually rather long, in some tropical forms 

 (Diadema) even about a foot long ; usually they are cylindrical, 

 tapering, but they may be greatly thickened, egg-shaped, or com- 

 pressed. In the irregular Echinoids they are partly transformed 

 into flat, spade-shaped digging apparatus, while others are very 

 small, club-shaped, close set, forming peculiar bands, the 

 " fascioles ", which have a definite course, different in the various 

 forms and therefore of great systematic importance. While in 

 the irregular Echinoids the spines are, as a rule, small, very 

 numerous and crowded, without definite order, they are in the 

 regular Echinoids usually large, rather few, and regularly 

 arranged. Generally there is a larger spine in the centre of each 



plate, and these primary sjnnes then 

 form a conspicuous longitudinal series 

 to each series of plates. The smaller, 

 secondary spines may also form regular 

 longitudinal series, mainly at the 

 circumference, the " ambitus ", of the 

 test, where, as a rule, the tubercles 

 Fig. 142.-P^S^ate, crenu- ^nd spines are most strongly de- 

 late tubercle of anEchinoid veloped. A corresponding arrange- 

 (EchinotMrix Desori) x (5 j^^j^^ ^f course, is shown bv the 

 {From JJan77iark s r aiaia.) t^ " n, 



a, The areole ; cr, The crenulate tubercles On the naked tCSt (Fig. 139). 



edge of the platform ; k, The ma- ^he pedicellarice (Fig. 143) are 



melon ; ke, The tubercle cone, the -^ \ & / 



"boss"; p, The groove or pit in the small, delicate Seizing organs, which 



mamelon. pi i i 



are found scattered among the spmes, 

 and, like these latter, are articulated to tubercles on the test, 

 these tubercles being, however, very small, much smaller than 

 the spine-bearing tubercles. They consist of a head and a stalk, 

 to which latter the head is attached either directly, through 

 muscles, or through a long, very flexible neck. The head consists 

 of usually 3 valves, more rarely only 2 or 4, or even more, each 

 with its very characteristic calcareous skeleton (Fig. 144). The 

 3 valves are connected with each other by strong muscles, and 

 are thus able to open and close. On the inner side of each valve 

 is found, as a rule, a smafl elevation provided with fine sensory 

 hairs. If anything touches these hairs, the valves close instantly. 

 The pedicellariae are a sort of weapons of defence, which seize any 

 foreign organism that comes into contact with the animal ; they 

 do not release their hold as long as the object moves, and if it is 

 too strong to be held, the pedicellarise are torn from the test of 



