SHELL AND SPINES OP ECHINIDA. 457 



in the living state, is softer and more spongy than in the lower 

 portion ; and it is in its substance that the first deposition of 

 calcareous matter takes place, which constitutes the origin of a 

 new plate. A careful examination of a specimen in progress of 

 growth will show plates in all stages of development, from the 

 thin calcareous scale on which no rudiment of spines can be seen, 

 to the solid plate furnished with bold projections, to which are 

 attached spines several inches long. The structure and origin 

 of these spines will be presently described. 



1094. The shell of the Echinida is covered in the living state 

 by spines, which are sometimes very short and delicate, in other 

 instances long and slender, and in many species of moderate 

 length and considerable massiveness. These spiues are attached 

 to tubercles upon the surface of the shell, which are usually large 

 in proportion to the size of the organs they support. The tuber- 

 cles do not cover the whole surface, however ; but they are 

 disposed in regular rows from one opening of the shell to the 

 other ; and the plates from which they* are developed are called 

 tubercular plates. Between the bands of tubercles there are 

 others almost destitute of these prominences ; and the pieces 

 of the shell composing them are termed ambulacral plates. 

 These plates are usually not altogether destitute of tubercles ; 

 but the tubercles, as well as the spines which they bear, are very 

 small in proportion to the same parts on the other series. The 

 ambulacral plates are perforated by a large number of very 

 minute orifices, through which delicate membranous tubes are 

 put forth ; these assist the animal in walking, or seizing its 

 prey, in the manner to be presently described. 



1095. The spines are united to the shell by the general 

 enveloping membrane, which attaches itself round their bases ; 

 a portion of this takes the form of a muscular sheath enclosing 

 the base of the spine and the tubercle, and by its contractions 

 the spines are caused to move in any required direction, 

 their cup-like bases working upon the tubercles, in the man- 

 ner of a ball-and-socket joint. In the Cidaris, which may 

 be regarded as the most perfect of the Echinida, the spines are 

 also attached by a round ligament (exactly like that of the hip- 



