Scientific Lectures. 27 



All true microscopic animals, like other animals, may be seen to 

 ingest and digest their food ; and this food consists always of other 

 animals which are their natural prev, or of the lower orders of vege- 

 table life ; -^vhile vegetables draw their support from water, carbonic 

 acid and ammonia, and set free oxygen under the influence of the 

 light of day. These animals, therefore, cannot live in waters perfectly 

 pure, nor in waters which contain only mineral impurities ; and a 

 knowledge of this fact may serve to relieve'the minds of those whom 

 the unguarded statements of some writers have led to believe that 

 all waters are at all times swarming with animalcular life. In the 

 waters of the Croton as it reaches our dwellings, I have rarely 

 detected any living thing, either animal or vegetable. In the exam- 

 inations of this water, constantly kept up under the direction of our 

 Board of Health, traces of life are occasionally detected ; but, per- 

 haps, it is well enough to observe here, that should any source of 

 water be anywhere found to be absolutely, and at all times free from 

 every indication of the presence in it of microscopic life, the pre- 

 sumption would be rather unfavorable to its wholesomeness than the 

 contrary ; since water which will not sustain animalcular life is 

 hardly fit for man ; and water which will do so will never be wholly 

 and permanently without it. 



Besides the distinctions here pointed out between the humbler 

 forms of animals and vegetables, there is a second which seems to be 

 no less characteristic. The cell-wall of the unicellular animal is sin- 

 gle, while that of the unicellular vegetable is double. The inner wall 

 of the vegetable cell resembles the wall of the animal cell, and is 

 albuminous in its nature. The external wall is of the nature of 

 cellulose, a substance never found in any part of any animal, but 

 which has an elementary composition resembling that of starch, and 

 in physical properties is best illustrated by the fiber of cotton. The 

 two walls of the vegetable cell often adhere 83 closely as to be 

 separated only with extreme difficulty, but there is no reason to 

 doubt that they are always present. 



From what has just been said, it will be perceived that the distinc- 

 tion between microscopic plants and animals is not one which is 

 obvious to mere inspection, or, indeed, one which can be certainly 

 detected Mdthout patient observation. And the difficulty is height- 

 ened by the fact that while we see plants exhibiting apparently all 

 the visible characteristics of animals, we see animals also assuming 

 the forms and singularly simulating the habits of plants. The vorti- 



