38 6 THE MICROSCOPE. 



factory conclusions upon the habits of an animal thai 

 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 rnollusca, 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 mariner generally succeeds at last in 

 barring him out. 



Skeletons of Sponges. The skeletons of sponges, which 

 give shape and substance to the mass of sarcode that con- 

 stitutes the living animal, is best made out by cutting thin 

 slices of sponge submitted to firm compression, and view- 

 ing these slices mounted upon a dark ground, or backed up 

 with black paper. 



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, and more or less cylindrical, threads of a light- 

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

 t f hey frequently aiiastamose, and vary considerably in size ; 

 when developed to a great extent, needle-shaped siliceous 

 bodies termed spicula (little spines) are formed in their in- 

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

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

 sponges, as those belonging to the genus Halichondria, the 

 name horny kind of material is present in greater or less 

 abundance ; but its fibrous structure has become obscure : 



