286 THE MICROSCOPE. 



factory conclusions upon the habits of an animal that 

 works so completely in the dark as the Cliona celata 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 excavating power to the presence 

 of a multitude of minute siliceous crystalline 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 disintegration of sub- 

 marine calcareous bodies the shelly coverings, it may be, 

 of animals far higher in organisation than they ; nay, in 

 many instances they prove themselves formidable enemies 

 even to living mollusca, by boring completely through the 

 shell. In this case the animal whose domicile is so unce- 

 remoniously invaded, has no alternative but to raise a wall 

 of new shelly matter between himself and his unwelcome 

 guest ; 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 some of 

 them are partially formed before the birth of the polypes 

 which have to become their inhabitants. 



The skeletons of sponges are composed principally of 

 two materials, the one animal, the other mineral ; the first 

 of a fibrous horny nature, the second either siliceous or 

 calcareous. The fibrous portion consist of a network of 

 smooth, arid more or less cylindrical threads of a light- 

 yellow colour, and, with few exceptions, always solid; they 

 frequently anastamose, and vary considerably in size ; 

 when developed to a great extent, needle-shaped siliceous 

 bodies termed spicula (little spines) are formed in their 

 interior ; in a few cases only one of these spicula is met 

 with, but most commonly they occur in bundles. In some 

 sponges, as those belonging to the genus Halichondria, the 

 same horny kind of material is present in greater or less 

 abundance ; but its fibrous structure has become obscure : 

 the fibres, however, in these cases are represented by sili- 



