186 THE MICROSCOPE. 



ment of the largest, and they appear to be endowed with the power of 

 locomotion in proportion to their size." 



Not the least wonderful circumstance connected with the history of 

 the sponges is the power possessed by certain species of boring into 

 substances, the hardness of which might be considered as a sufficient 

 protection against such apparently contemptible foes. Shells, both 

 living and dead, coral, and even solid rocks, are attacked by these 

 humble destroyers, gradually broken up, and, no doubt, finally re- 

 duced to such a state as to render substances which would otherwise 

 remain dead and useless in the economy of nature available for the 

 supply of the necessities of other living creatures, 



These boring sponges constitute the genus Cliona, and some allied 

 genera. They are branched in their form, or consist of lobes united by 

 delicate stems ; they all bury themselves in shells or other calcareous 

 objects, preserving their communication with the water by means of 

 perforations in the outer wall of the shell. The mechanism by which a 

 creature of so low a type of organisation contrives to produce such 

 remarkable effects is still doubtful, from the great difficulties which 

 lie in the way of coming to any satisfactory conclusions upon the 

 habits of an animal that works so completely in the dark as the Cliona 

 it will probably long remain so. Mr. Hancock, to whom we are 

 indebted for a valuable memoir upon the boring sponges, published in 

 the Annals and Magazine of Natural History, attributes their exca- 

 vating power to the presence of a multitude of minute siliceous crystal- 

 line particles adhering to the surface of the sponge ; these he supposes 

 to be set in motion by some means analogous to ciliary action. In 

 whatever way this action may be produced, however, there can be 

 no doubt that these sponges are constantly and silently effecting the dis- 

 integration of submarine calcareous bodies the shelly coverings, it may 

 be, of animals far higher in organisation than they ; nay, in many in- 

 stances they prove themselves formidable enemies even to living mol- 

 lusca, by boring completely through the shell. In this case the animal 

 whose domicile is so unceremoniously invaded, has no alternative but 

 to raise a wall of new shelly matter between himself and his unwelcome 

 guest j and in this manner generally succeeds at last in barring him out. 



SKELETONS OF SPONGES. 



The skeletons of Sponges, as well as those of Zoophytes, possess no 

 blood-vessels ; they are secreted by the fleshy mass of the animal, and 



