STRUCTURE OF SPINES. 



193 



Fig. 95. 



double rows upon the inner surface of the ambulacral pieces*, by the 

 intervention of which they are connected with the canals above men- 

 tioned. 



(507.) The tubercles upon the external surface of the shell of the 

 Echini support a corresponding number of long spines, which, as well as 

 the apparatus of suckers, are employed as locomotive agents. These 

 spines vary materially in their form and proportionate size, and even in 

 their internal structure 

 and mode of growth, as 

 may be readily seen by 

 a comparison of different 

 species. Thus, in the 

 flattened forms of Scu- 

 tellce and allied genera, 

 they are so minute as to 

 require the employment 

 of a microscope for their 

 investigation ; in Echi- 

 nus esculentus (fig. 85) 

 they are sharp, and al- 

 most of equal length 

 over the entire surface 

 of the animal ; while in 

 the specimen repre- 

 sented in the annexed 

 figure (fig. 95), the shell of which we have already examined when 

 divested of these appendages, the length of the spines that are articu- 

 lated upon the large tubercular plates fully equals the transverse dia- 

 meter 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, allowing of a considerable extent of motion. The 

 structure of this articulation is exhibited in fig. 94, 2. The large 

 tubercle (a) supports upon its apex a smaller rounded and polished 

 eminence, perforated in the centre by a deep depression ; and the bottom 

 of the moveable spine (c) is terminated by a smooth hemispherical cavity 

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

 plete 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 the round ligament found in the hip- 

 joint, and obviously a provision for the prevention of dislocation. 



* Cyclopaedia of Anat. and Phys., art. " ECHINODERMATA." 







Cidaris. 



