PROTOZOA AND SPONGES. 885 



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 preyious evening, none 

 were visible on the glass. Thus they must crawl, on oc- 

 casion, from a hundred 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 sarcode 

 is invested, but consists of fibres or points or rods of vary- 

 ing form, which 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 anasto- 

 mose in every direction, at very short intervals, 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 continuity 

 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 multitude of glassy needles, all 

 separate and independent, between themselves, yet so con- 

 trived that they do hold together very firmly, and in a 

 great number of cases are arranged on a prescribed plan, 

 so as to give a certain form and outline to the aggregate. 



If you have ever shaken up a box of dressing-pins, and 

 have then endeavoured to take one out, you know how by 

 their mere interlacement they adhere together in a mass, 



* A group of animals with shells, resembling in appearance those of 

 the common nautilus ; and, like them, consisting of several chambers 

 divided from one another by walls which are pierced with numerous 

 minute holes, in Latin called foramina ; whence their name. 

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