SPINES OF ECHINIDA. 449 



joint in Man), which is inserted into a little depression at the 

 top of the tubercle, and also into a small hollow in the bottom 

 of the cup. In this manner great freedom of motion is pre- 

 served ; and that degree of strength is added, which is required 

 for the vigorous actions performed by the solid massive spines 

 of this animal. The general enveloping membrane serves not 

 only for the movement of the spines, but also for their nutrition. 

 During the formation of the plates, as just now described, they 

 shoot out from its surface like horns from the head of a stag. 

 They remain covered, however, by delicate prolongations of the 

 living membrane ; and by these new layers are deposited, one 

 around the other, so that the diameter and the length of the 

 spine are increased at the same time. The structure of the 

 spines corresponds with that of the shell ; and it is remarkable 

 for the exquisite beauty and regularity of its arrangement, as is 

 seen when a very thin transverse section is placed under the 

 Microscope. The degree in which the spines are employed as 

 organs of locomotion, or for other purposes, will vary, of course, 

 with their size and strength. In the Cidaris, they seem to act 

 as so many legs ; and they are also used for excavating hollows 

 in the sand, upon which the animal lies. In many of the Echini, 

 they would appear too slender and fragile to support the weight 

 of the body ; but it is to be remembered that, when it is im- 

 mersed in water, the pressure upon them will be very small. In 

 some, however, they are broad and flat ; and enlarge towards 

 their extremities, instead of tapering to a point ; the animals 

 possessing these can move themselves in any direction by means 

 of them, even on land. In the Scutella^ the spines are extremely 

 minute, but, at the same time, vastly increased in number ; each 

 of them is as perfectly organised, and its motions as much 

 governed by the animal, as when they are fewer and larger ; 

 and by means of their combined action, the animal can speedily 

 bury itself in the sand, for the purpose either of procuring food 

 or of escaping from its enemies. 



1006. In many of the Echinida, however, the chief locomo- 

 tive organs are of a character entirely different. From the 

 openings in the ambulacral plates, a number of very delicate 



