ECHINID^E. 



249 



ing among the rocks, where they entrap their prey. The spines and 

 suckers are used as feet, or as a mode of progression, even to the 

 climbing of rocks, in order to feed upon the corallines and zoophytes : 

 he marches along with ease where apparently no footing could be found, 

 or digs a hole with his spines to bury himself in the sand, to escape 

 his pursuer, or to hide himself from observation. 



The skeleton of these animals generally consists of an assemblage 

 of plates, or joints, of calcareous matter. The minute structure of 

 which presents more or less a reticu- 

 lated character ; and the solid parts are 

 usually composed of a series of super- 

 imposed lamina? or scales. The open- 

 ings, or areolse, in one layer being 

 always placed over the solid cell- walls 

 of the layer beneath it, the spines are 

 situated on the external surface of the 

 shell ; they are generally of a conical 

 figure, and are articulated to the tu- 

 bercles by a ball-and-socket joint. 

 When a thin transverse section of one 

 of these spines is examined with the 

 naked eye, it appears to be made up of 

 a series of concentric layers, varying 

 considerably in number ; not, how- 

 ever, with the size of the spine, but 

 with the distance from the base at 

 which the section was made ; when a 

 section taken from the middle of the 

 spine is examined with a power of fifty 

 diameters, it will be seen that the 

 centre is occupied by a reticulated 

 structure ; around the margin of this 

 may be observed a series of small 

 structureless spots, arranged at equal 

 distances apart (Plate VI. No. 2) ; these 

 are the ribs or pillars, and indicate the 

 external surface of the first layer de- 

 posited; passing towards the margin, 

 other rows of larger pillars may be 

 seen, giving it a beautiful indented appearance ; all the other parts of 

 the section are occupied by the usual reticulated tissue. In the 



fig. 108. 



Polypidom of Pennatula phos- 

 phorea. 2. Synapta. 3. Anchor- 

 shaped spiculum and plate from 

 its skin. 



