1 6 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Thus the form (1) of a plant is diffuse and arbores- 

 cent, that of an animal oblong and rounded. A plant 

 lives on (2) carbonic acid and mineral salts, but an 

 animal requires albuminoid foods. These foods are in 

 the plant taken in (3) by the porous tissues, and there 

 is no distinct mouth as there is in all but the lowest, 

 and in the majority of parasitic animals. The secre- 

 tions (4) of a plant are non-nitrogenous, while some of 

 the waste products of an animal always contain 

 nitrogen. In their habits (5) we find that plants are 

 fixed, and animals locomotive. And, lastly, (6) as to the 

 characters of their cells, we find that plants have a cell 

 wall formed of that ternary compound which is known 

 as cellulose, while the wall of an animal cell, when 

 present, is derived directly from the cell protoplasm. 



To nearly all the statements now made an ex- 

 ception may be found : thus (1) cacti and fungi are 

 certainly not arborescent or diffuse, while polyps as cer- 

 tainly are. (2) Fungi appear to require some more 

 complex compound than merely carbonic acid and 

 mineral salts, but such a body as ammonium tart rate 

 will give the nourishment required ; every animal 

 known to us requires albuminoid food, and dies when 

 deprived of it. (4) It is quite true that plants do 

 not give off nitrogenous excreta ; but their protoplasm, 

 it must always be remembered, is capable of forming 

 them ; on the other hand, all the excreta of an animal 

 are not nitrogenous; Ascidians (and, if they are 

 truly animals, some of the Cilio-flagellata) form cellu- 

 lose. The latter and some low worms have been ob- 

 served to form starch, and sugar is a ternary compound 

 formed by various animals. The well-known Yolvox 

 offers (5) an exception to the statement that plants are 

 fixed, and polyps and, to a large extent, stalked Echi- 

 noderms, to the statement that animals are locomotive. 

 Lastly, some of the lowest plants, such as Myxomycetes, 

 have their protoplasm naked, while the just-mentioned 



