52 THE LIVING SPONGE. 



muzzle, etc. and also in the arrangement and colour- 

 ing of the hairs, spines, scales, or feathers. But 

 this definiteness of figure, involving a constancy in 

 the number and arrangement of composing parts, 

 diminishes in degree as we verge towards the lower 

 groups, and when we arrive at the lowest, we see 

 diversity in the midst of sameness. 



Nor can we be surprised at this, when we reflect 

 upon the condition of the nervous system, and upon 

 the vital laws by which these beings are governed. 

 No two sponges of the same species correspond in 

 figure, or in the number of their canals ; for the 

 latter multiply as the animal increases, and circum- 

 stances influence development in one part more 

 than another. 



But though this be the case, there are limits to 

 the law of variation, so that no species puts on the 

 appearance of another. The cup sponges never 

 approach such as are branched, nor these such as 

 resemble tufts of moss ; though two cup sponges 

 are never precisely of the same outline, nor two 

 branched sponges of the same shape, and with 

 the same figure and proportion of their ramifica- 

 tions. 



Dr. Grant put a small branch of a sponge, with 

 some sea-water, into a watch glass under the micro- 

 scope, and on reflecting the light of a candle 

 through the fluid, he perceived that there was some 

 inward motion in the opaque particles floating 

 through the water. On moving the watch-glass, so 



