ECHINODERMA. 1 1 9 



parts are almost entirely confined by a fragile and inflexible globular 

 crustj this is never shed and reproduced, like the shells of the crab 

 and lobster. At the same time the calcareous plates possess not more 

 power of inherent growth than the crusts of the Crustacea, which 

 they resemble in both physical and chemical properties. By the sub- 

 division of the hollow globe into many pieces, and the apposition of a 

 formative membrane to all their margins, addition is gradually made 

 to the circumference of each component plate, and by the plan of 

 their arrangement the spheroidal shell gradually expands, with little 

 change in its figure and relative proportions. 



The amount of change in the form of the shell, which differs in 

 different species, depends upon the addition of new plates to the 

 ambulacral and interambulacral series. These are developed near 

 the oral and anal poles, but chiefly near the latter, where, in the 

 young Cidaris, for example, the plates are more loosely connected 

 together, and support incomplete spines. In the membrane con- 

 necting such plates may be seen small irregular pieces, without tu- 

 bercles or spines, which gi'ow by accretion to their margins, and then 

 have the tubercles developed upon their outer surface. The spines 

 are at first immoveable, and stand out like processes from the tuber- 

 cle ; the joint is not developed until after they have acquired a certain 

 size. The growth of the globe in the direction of its poles is chiefly 

 by the development of the new plates ; its expansion at the equator is 

 by the addition to the sutural margins of the old plates. 



The spines of the Echini vary in form and relative size in different 

 genera ; their proximal extremity is adapted, by an excavation, to the 

 tubercles on the outer surface of the plate, to which it is attached by 

 a capsular ligament, and upon which it can be rotated by muscular 

 fibres external to the capsule. In the species of Cidaris, where the 

 spines are unusually large, an internal ligament extends from a little 

 pit upon the centre of the tubercle to the centre of the articular 

 cavity of the spine, analogous to the round ligament in the hip joint. 

 The spines grow by successive additions, through calcification of that 

 part of the common organised membranous covering of the shell of 

 the Echinus, which is attached to their base. The varied cellular 

 organisation of the spines, affords beautiful microscopical objects, 

 when viewed in thin transverse slices. 



The tubes that issue from the ambulacral pores can be extended 

 beyond the longest spines in the Echinus Sphccra of our own coasts ; 

 they terminate in suckers, which appear to be highly sensitive, and by 

 which the Sea-urchin attaches itself to foreign bodies, and moves 

 along them with a rotatory course, in which the spines *serve to 

 balance and direct the progress of the animal. The bases of the 



I 4. 



