206 FUNCTIONS AND INSTINCTS. 



their motions, found, that whether they moved in 

 a horizontal position, as they usually do, or in a 

 reversed one, or upon their sides, they princi- 

 pally used their spines. As they can move in any 

 direction, some are used as legs for progressive 

 movement, others as points of support to prevent 

 a retrogressive one. It is by means of their 

 spines, also, some performing one office and some 

 another, that they bury themselves in the moist 

 sand on the sea shore. 2 



It is not easy to conceive by what mechanism 

 the spines are moved ; the protuberances on 

 which they move are fixed, and there appears to 

 be no communication between the interior of the 

 shell and the membranous sac by which they 

 are attached to them. " It is very difficult," 

 says Cuvier, " to see the fibres that move these 

 spines at the will of the animal, for nothing is 

 observable in their articulation but a very solid 

 ligamentous substance, which it is very difficult 

 to cut. I have examined, with a lens of con- 

 siderable power, the shell both within and 

 without, and have been able to discover no 

 pores on either side, round the base of the 

 protuberances or elsewhere ; so that it seems 

 impossible for any muscular threads, however 

 fine, to pass from the body of the animal to 

 the connecting ligament by which it could move 

 it and so give the spine its different inclinations. 



1 Osier in Philos. Tr. 1826. 



