172 GENERAL PRINCIPLES OF ZOOLOGY. 



Physiological Distinctions. — If one wish to discover sharp 

 distinctions between animals and plants, he may take into con- 

 sideration on the one side physiological, on 

 tlie other morplujlogical, characters. Start- 

 ing from the jihysiological point of view, 

 Liunaius ascribed to plants only the capacity 

 of reproduction and nutrition, but to animals 

 the jiower of motion and sensation in addi- 

 tion. However, we know that vegetable, 

 like animal, protoplasm is irritable and is 

 caj^able of movement, as is shown by the 

 active movements of the lower Algae, the 

 great sensitiveness of the Mimosa, and other 

 FiQ. lu.— Lepas anati- plants; but further, we know that even many 



fera. (After Schmar- „,, i-ni -n -i 



da.) c, carina; t, ter- of the more highly Organized animals, e.g., 

 gum, s, &CU um. Crustacca (fig. 114), lose the power of loco- 



motion and become fixed, and many fixed forms, e.g., the spionges 

 (fig. 84), even under the closest examination appear immovable 

 and unaffected by stimulation; thus we are led to abandon the 

 idea that the so-called animal functions are to be regarded as 

 accurate distinctions. 



Metabolism not a Safe Criterion. — Even the difference in met- 

 abolism is by no means sufficient. Every plant has a double 

 exchange of material. In its movements and other vital functions 

 the vegetable protoplasm produces carbon dioxide and consumes 

 oxygen; at the same time there goes on, under the influence of 

 sunlight and of chlorophyl, the reduction of carbon dioxide and 

 the giving off of oxygen. In chlorophyl-containing plants the 

 reducing process preponderates so considerably during the day 

 that there is evident, as the final result, the giving off of a greater 

 quantity of oxygen, and only at night, when the reducing jjrocess 

 becomes interrupted on account of the lack of sunlight, does the 

 production of carbonic-acid compounds become perceptible. But 

 the reducing processes become immediately preponderant if the 

 chlorophyl be absent; chlorophylless moulds and bacteria have, 

 therefore, the same metabolism, so far as carbon dioxide is con- 

 cerned, as animals. 



Cellulose not a Sure Test. — So also it is incorrect to sav that 

 only plants have the power to make cellulose, for cellulose is found 

 in many lower animals, the rhizopods, and in the highly organized 

 group of tunicates; according to recent investigations it appears 

 to be found even among the arthropods. 



