

SHELLS OF MOLLUSCA. 



167 



whole substance, separating from each other the parts formerly de- 

 posited, but by the mere addition of particles to its surface and extre- 

 mities. In this manner the growth of a solid Coral-structure may 

 go on to an enormous extent ; the surface at which the consolidating 

 action is going on, being the only part alive, that is exhibiting any 

 vital change ; and all the rest of the mass being henceforth perfectly 

 inert. 



278. In the class of Echinodermata, which includes the Star-fish, 

 Sea-Urchin, &c., we find the calcareous structure presenting a very 

 elaborate organization ; as an example of this, we shall select the shell 

 of the Echinus, commonly known as the Sea-Egg. This shell is made 

 up of a number of plates, more or less regularly hexagonal, and fitted 

 together so as completely to enclose the animal, except at two points, 

 one of which is left open for the mouth, the other for the anus. On 

 the surface of these plates are little tubercles, for the articulation of the 

 spines, which serve as instruments of defence and of locomotion. The 

 substance of the shell and of the spines is exactly alike ; being a sort, of 

 areolar tissue, consolidated by the deposition of calcareous matter, and 

 having an innumerable number of interspaces or minute cancelli, freely 

 communicating with each other. The arrangement of this calcareous 

 network in the spines is most varied and elaborate ; and causes thin 

 sections of them to be among the most beautiful of all microscopic ob- 

 jects. The external and internal surface of each plate, in the shell of 

 the living Echinus, is covered with a membrane, from which its nutrition 



Fig. 42. 



Portion of the shell of the Echinus, showing at a the constituent plates, and at b the calcified 

 areolar tissue, of which they are composed. 



is derived ; this membrane dips down into the spaces between the ad- 

 jacent plates ; but it does not penetrate the substance of the plates 

 themselves, nor does it transmit vessels to their interior. A simi- 

 lar membrane covers and encircles the spines; and it also connects 

 these with the shell, being continuous with the membrane that envelopes 

 the latter. Thus each plate and spine is itself completely extra-vascu- 

 lar ; but it is enclosed in a soft membrane, which furnishes (whether 

 by vessels or otherwise, has not yet been ascertained), the elements of 

 its nutrition. 



279. But we do not here find any evidence of interstitial growth ; 

 nor is there any reason why such should be required. For the tissue 



