COMPLEX ArPARATUS FOR NUTRITION. Ill 



because nearly all the inferior tribes subsist 

 wholly upon animal substances. Many of these 

 latter animals have organs capable of extracting 

 nourishment from substances, which we should 

 hardly imagine contained any sensible portion 

 of it. Thus, on examining the stomach of the 

 earth-worm, we find it always filled with moist 

 earth, which is devoured in large quantities, for 

 the sake of the minute portion of vegetable and 

 animal materials that happen to be intermixed 

 with the soil ; and this slender nutriment is suf- 

 ficient for the subsistence of that animal. Many 

 marine worms, in like manner, feed apparently 

 on sand alone ; but that sand is generally in- 

 termixed with fragments of shells, which have 

 been pulverized by the continual rolling of the 

 tide and the surge ; and the animal matter con- 

 tained in these fragments, affords them a supply 

 of nutriment adequate to their wants. It is evi- 

 dent, that when, as in the preceding instances, 

 large quantities of indigestible materials are 

 taken in along with such as are nutritious, the 

 stomach and other digestive cavities must be 

 rendered more than usually capacious. It is 

 obvious also that the structure of the digestive 

 organs must bear a relation to the mechanical 

 texture, as well as to the chemical qualities of the 

 food ; and this we find to be the case in a variety 

 of instances, which will hereafter be specified. 

 The activity of the digestive functions, and the 



