40 PHYSIOLOGICAL ILLUSTRATIONS. 



with an apparatus of hair, calculated to repel that fluid ; hence gnats and certain other 

 insects have no difficulty in passing over the surface of water. By the same means the 

 hydra suspends itself, without effort, in that element ; for having exposed for a time the 

 extremity of its foot to the air, so that it may become dry, it, by repulsion, forms a cup- 

 shaped hollow around it, the head of the insect hanging down in the water beneath. 



130. Organs of exhalation and absorption are unquestionably capillary systems. 

 The stomata of plants, which botanists suppose to discharge these functions, are of this 

 character ; they furnish a well-marked instance of the accommodation of apparatus to 

 suit physical conditions. Plants growing beneath the surface of water have no stomata ; 

 but if, by any means, they reach the atmosphere and vegetate in it, these organs are pro- 

 duced for the purpose of discharging, under the new order of things, offices which were 

 accomplished by other means. The spongioles of roots, acting as capillary systems, 

 drive the fluids they absorb from the earth, through the tubular vessels of trees, with a 

 force of several atmospheres, extending themselves at a due distance from the trunk, 

 where they may meet with the water that falls from the leaves. In some orders of 

 living things, which are not accommodated with distinct orifices for the reception of 

 food, nutrition is accomplished by capillary systems. In this manner the porifera ex- 

 pose a wide surface to the seas, and draw in nutrient matter through their microscopic 

 pores, discharging the surplus, as excrementitious matter, through their papillary orifices. 



131. Like the lungs of the mammalia, the leaves of trees are respiratory organs, com- 

 posed of capillary systems ; their mechanical functions are not so complete, though 

 their chemical functions may be identical. They demand no nervous cords to be spread 

 upon them to give them motion and keep up their play ; the breezes in which they 

 tremble perform the office of carrying off the exhaled impurity ; and the rays of the sun 

 furnish them with their vital force, enabling them to effect the decomposition of carbonic 

 acid, and provide a store of carbon for the purposes of the economy. 



132. In identifying the mechanical with the chemical force of organic structures, we 

 see another proof of that unity of design existing through the entire range of living 

 things. Functions of all kinds are accomplished by arrangements of every sort in dif- 

 ferent classes, yet no one will deny that they all follow one original type. Digestion, 

 as it takes place in the stomach of man, appears a highly complex phenomenon, de- 

 pending, as some say, partly on the tissue action, partly on nervous, and partly on oth- 

 er powers. But are not analogous changes wrought without all this complexity of ap- 

 paratus in the hydatid, which may be taken as the elementary type of the stomach ; or 

 in the taenia, which is a colony of stomachs 1 The polygastric infusorials, some of 

 which have hundreds of these organs, and even the mammalia, do not digest more per- 

 fectly than the hydra, a carnivorous polypus, which may be turned inside out without 

 detriment. The laws of digestion followed by the one, are followed, too, by the other. 

 If the organ of the one respects the presence of living matter, and refuses to act on it, so 

 does that of the other ; yet the one is furnished with a highly complicated assemblage of 

 muscular bands, of glandular apparatus, of bloodvessels, of nerves, and the other is not. 



133. In the higher orders of life, processes are carried on by multiplied apparatus, 

 without, however, deviating from the principle of the original simple type. The gift of 



