122 



ECHINODERMA. ECHINOIDEA 



-b 



—a 



• c 

 ■h 



Fig. 49. 



on the boss (b), the upper margin of which is sometimes 



smooth, sometimes crenulated. The base of the boss is 



frequently surrounded by a 



smooth excavated space, the 



areola or scrobicule (a), to 



which muscles from the spine 



are attached. The granules 



are smaller than the tubercles 



and have no distinct mame- 



lon. 



Attached to the tubercles 

 are the spines or radioles ; 

 these are of different sizes 

 and shapes in different genera 

 and species and even on the 

 same individual,being needle- 

 like, rod-like, or flask-shaped; 

 the larger spines are attached 

 to the primary tubercles, the 

 smaller to the secondary tubercles. They serve for 

 protection and also assist in locomotion. At the end 

 of the spine, where it articulates with the mamelon, 

 there is a rounded cavity, the acetabulum (fig. 49 A, a) ; 

 next comes the head (h) limited above by a ring or collar (c), 

 which may be smooth or crenulated and serves for the 

 attachment of the muscles that move the spine. Beyond 

 the collar and forming the greater part of the spine is the 

 shaft or stem (b), which may be smooth, or ornamented 

 with ridges or rows of spiny processes. The microscopic 

 structure of the spines (fig. 41 A) varies in different 

 genera, and is of some importance in classification. 

 Pedicellarise, which consist of a stalk with usually three 

 blades, also occur, but are rarely found fossil. 



A. Spine of Cidaris 

 Jiorigemma, from the Corallian 

 Rocks, a, acetabulum ; h, head 

 or base ; c, collar ; b, shaft or 

 stem. B. Ambulacral plate of 

 Cidaris (recent) with a large 

 primary tubercle and secondary 

 tubercles. In the primary 

 tubercle, in, mamelon ; b, boss ; 

 a, areola. Natural size. 



