1:40 EYEXIXGS AT THE MICKOSCOPE. 



changes of position in the shell while under obser- 

 vation, is, that it is by means of the adhesion and con- 

 traction of t\\Q pseudopodia that the animal drags itself 

 along a fixed surface. This it can effect so assiduously, 

 that I frequently find them in the morning adhering to 

 the tank-sides three or four inches from the bottom, 

 though on the previous evening none were visible 

 on the glass. Thus they must crawl, on occasion, from 

 a hundi-ed to a hundred and fifty times their own 

 diameter in a night. 



The structure of a Sponge is much the same as that 

 of these animals, with the exception that its solid part 

 or skeleton is not a continuous covering by which the 

 sarcole is invested, but consists of fibres or points 

 or rods of varying form, Avhich are clothed with 

 the sarcode. This loose sort of skeleton may be of 

 horny or chitinous matter, like that of Arcella, or 

 calcareous, like that of the Foraminifera^ or it may 

 be siliceous — that is, composed of flint {silex). 



In some cases, as in the common Turkey Sponge, 

 the horny skeleton consists of a network of solid but 

 slender fibres, very tough and elastic, which branch 

 and anastomose in every direction, at very short inter- 

 vals, as you may see by looking at this atom, which I 

 cut off from a dressing sponge. 



In the lime and flint Sponges, however, the con- 

 tinuity and cohesion of the skeleton does not depend 

 upon the organic union of the constituent parts, as it 

 does in the loose and open network of the Turkey 

 sponge. For it is made up of an immense midtitude 

 of glassy needles, all separate and independent, between 

 themselves, yet so contrived that they do hold together 

 very firmly, and in a great number of cases are arranged 



