164 ECHINODERMATA. 



growth, as may be readily seen by a comparison of different 

 species. Thus, in the flattened forms of Scutella and allied 

 genera, they are so minute as to require the employment of a mi- 

 croscope for their investigation ; in Echinus esculentus (Jig. 62) 

 they are sharp, and almost of equal length over the entire surface 

 of the animal; while in the specimen represented in the an- 

 nexed figure (Jig. 69), the shell of which we have already 



Fig. 69. 



examined when divested of these appendages, the length of the 

 spines which are articulated upon the large tubercular plates fully 

 equals the transverse diameter of the body of the creature, and in 

 some cases they are even found much more largely developed. 

 Every spine, examined separately, is seen to be united with the 

 tubercle upon which it is placed by an apparatus of muscular and 

 ligamentous bands, forming a kind of ball-and-socket joint, allow- 

 ing of a considerable extent of motion. In^/zg. 68, 2, the structure 

 of this articulation is exhibited. The large tubercle (a) supports 

 upon its apex a smaller rounded and polished eminence, perforated 

 in the centre by a deep depression : the bottom of the spine, 

 moreover, (c) is terminated by a smooth hemispherical cavity 

 accurately fitted to the projecting tubercle, so that the two form 

 complete articular surfaces. The bonds of union connecting the 

 spine with the shell are of two kinds : in the first place, there is a 

 stout ligament (a, c), extending from the little pit seen upon 

 the centre of the tubercle, to a corresponding depression visible 

 upon the articular surface of the spine, resembling very accurately 



