36 



LIFE, IN ITS LOWER FORMS. 



more rarely, the elegant Sack Sponge (Grantia compressa). 

 It takes the form of a little flattened bag of angular out- 

 line, and of a whitish hue, with an orifice at each angle. 

 The bags, which are frequently clustered, hang by a slender 

 base from the stalks of sea- weeds, or from the naked rocks 

 (PL II. fig. 3). When examined they are found to be 

 hollow, with thin walls ; and if a small portion be torn off 

 and placed beneath a microscope, it will exhibit well the 

 structure of a spicular Sponge. The substance will appear 

 crowded with, and almost composed of, calcareous crystals, 

 most of which are stars of three radiating points, but some 

 are linear needles, and on the exterior are many which are 

 pointed at one end, and terminate in a bent, club-like knob 

 at the other. 



It was this species which, under Mr Bowerbank's expe- 

 rienced eye and delicate manipulation, revealed the moving 

 cilia. By tearing specimens in pieces (for the use of the 

 keenest cutting instruments so crushed the texture as to 

 destroy the parts), and examining the separated edges with 

 high powers, he found that the sides are composed of a 

 number of hexagonal cells, denned by the peculiar arrange- 

 ment of the triradiate spicula, and having their walls 

 formed by a multitude of nucleated granules (fig. 4). 

 These angular cells are laid at right angles to the long 

 axis of the Sponge, extending from the outer surface to 

 the inner ; and they are crossed, near the middle, by a 

 thin partition, perforated in the centre. In this perfora- 

 tion, several long, whip-like cilia were seen lashing with 

 energy, and the same organs were afterwards found to be 

 connected with the granules of which the cell- walls were 

 composed. By means of the wavings of these cilia, then, 



