IO MANUAL OF ZOOLOGY. 



or Ascidian Molluscs ; and the so-called " glycogen," which is 

 secreted by the liver of the Mammalia, is closely allied to, if 

 not absolutely identical with, the hydrated starch of plants. As 

 a general rule, however, it may be stated that the presence in 

 any organism of an external envelope of cellulose raises a strong 

 presumption of its vegetable nature. In the face, however, 

 of the facts above stated, the presence of cellulose cannot be 

 looked upon as absolutely conclusive. Another highly charac- 

 teristic vegetable compound is chlorophyll, the green colouring- 

 matter of plants. Any organism which exhibits chlorophyll 

 in any quantity, as a proper element of its tissues, is most 

 probably vegetable. As in the case of cellulose, however, the 

 presence of chlorophyll cannot be looked upon as a certain 

 test, since it occurs normally in certain undoubted animals 

 (e.g., Stentor, amongst the Infusoria, and the Hydra viridis, 

 or the green Fresh-water Polype, amongst the Ccelenteratd). 



d. Motor Power. This, though broadly distinctive of ani- 

 mals, can by no means be said to be characteristic of them. 

 Thus, many animals in their mature condition are permanently 

 fixed, or attached to some foreign object ; and the embryos of 

 many plants, together with not a few adult forms, are endowed 

 with locomotive power by means of those vibratile, hair-like 

 processes which are called " cilia," and are so characteristic 

 of many of the lower forms of animal life. Not only is this 

 the case, but large numbers of the lower plants, such as the 

 Diatoms and Desmids, exhibit throughout life an amount and 

 kind of locomotive power which does not admit of being rigidly 

 separated from the movements executed by animals, though the 

 closest researches have hitherto failed to show the mechanism 

 whereby these movements are brought about. 



e. Nature of the Food. Whilst all the preceding points have 

 failed to yield a means of invariably separating animals from 

 plants, a distinction which holds good almost without excep- 

 tion is to be found in the nature of the food taken respectively 

 by each, and in the results of the conversion of the same. The 

 unsatisfactory feature, however, in this distinction is this, that 

 even if it could be shown to be, theoretically, invariably true, 

 it would nevertheless be practically impossible to apply it to 

 the greater number of those minute organisms concerning 

 which alone there can be any dispute. 



As a broad rule, all plants are endowed with the power of 

 converting inorganic into organic matter. The food of plants 

 consists of the inorganic compounds, carbonic acid, ammonia, 

 and water, along with small quantities of certain mineral salts. 

 From these, and from these only, plants are capable of elabo- 



