ECHINODERMATA. 201 



The small anal plates are united together like the oral ones by an 

 extensile and contractile membrane. Both the internal and external 

 surface of the rest of the complicated shell is covered by a similar 

 organised membrane, which likewise extends through all the numer- 

 ous sutures of the shell. "With this explanation of the general 

 structure of the crust of the Echinus we are in a condition to under- 

 stand the manner of its growth, which otherwise would be a difficult 

 physiological problem. 



The Echinus maintains nearly the same spheroidal figure from its 

 earliest formation to full maturity ; and, notwithstanding that its soft 

 parts are almost entirely confined by a fragile and inflexible globular 

 crust, 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 do the crusts of the Crustacea. By the 

 subdivision of the hollow globe into many pieces, and the apposition 

 of a formative membrane to aU their margins, addition is 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 grow 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. In fuU 

 grown and aged specimens, especially of Scutella and Clypeaster, the 

 sutures become obliterated. 



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

 genera ; in each the proximal extremity is adapted, by an excavation, 

 to one of 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 



