COMPLEX APPARATUS FOR NUTRITION. 09 



considerably greater than those required to convert 

 animal food into the same fluid ; because the latter 

 is itself derived, with only slight modification, im- 

 mediately from the blood. We accordingly find it 

 to be an established rule, that the digestive organs 

 of animals which feed on vegetable materials are 

 remarkable for their size, their length, and their 

 complication, when compared with those of car- 

 nivorous animals of the same class. This rule 

 applies, indeed, universally to Mammalia, Birds, 

 Reptiles, Fishes, and also to Insects ; and below 

 these we can scarcely draw the comparison, 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 por- 

 tion of vegetable and animal materials that happen 

 to be intermixed with the soil ; and this slender 

 nutriment is sufficient for the subsistence of that 

 animal. Many marine worms, in like manner, feed 

 apparently on sand alone ; but that sand is gene- 

 rally intermixed with fragments of shells, which 

 have been pulverized by the continual rolling of 

 the tide and the surge ; and the animal matter con- 



* Corda observed, that in the Hydra fusca the digestion of 

 animal substances is performed vvitli much greater rapidity than that 

 of vegetable ; and relates that ho saw a larva of an insect, with a 

 thick skin, disappear almost entirely, by being digested in the sto- 

 mach of the hydra, in four minutes ; whilst fragments of the plant 

 called Vaucheria clavnta were rejected by it unchanged. 



