26 POPULAR HISTORY OF THE AQUARIUM. 



chooses, or drops accidentally into, a suitable position for 

 its future growth, when once fixed it becomes a permanent 

 tenant of the spot. 



!N"o doubt can be entertained that the Aquarium, assisted 

 by the microscope, is destined to be the means of greatly 

 increasing our hitherto limited knowledge of these half- 

 animated beings. Mr. Bowerbank^s researches in the na- 

 tural history of the family have already been rewarded by 

 the discovery of many new forms, and by the elucidation of 

 parts of their economy. The large, spreading, fleshy TacJiy- 

 matimia JoJmstonii, with its thick skin studded with pores 

 few and far between, has been examined, and the curious 

 spicula (or stars and needles) of flint submitted to the micro- 

 scope. Mr. Bowerbank has also pointed out that the cur- 

 rents observed as entering the small pores and leaving the 

 large ones are produced by the motion of long fine cilia on 

 the inner surface of the cells, which are constantly vibrating 

 in the required direction, so that the means used for locomo- 

 tion during the extreme youth of the Sponge are the same 

 as those used for maintaining the principal functions of 

 vitality in the middle life and sedentary old-age of the same 

 creature. It appears too that as the horny skeleton of the 

 Sponge is the support of the very loose, gelatiuous animal 



