94 COMPARATIVE ZOOLOGY. 
growth. Ascending the scale of animal creation, we find 
the primitive form gradually obscured by new and power- 
ful molding influences. 
(4) Structure—A plant is the multiplication of the unit 
—a cell with a cellulose wall. Some simple animals have 
’a similar cellular structure, and all animal tissues, while 
forming, are cellular. But this character, which is perma- 
nent in plants, is generally transitory in animals. Their 
tissues are composed of little plates, or laminee, intersect- 
ing each other in such a way as to constitute spongy 
masses, or membranes, which are not made up of cells 
independent of each other, as in vegetables. Excepting 
the very lowest forms, animals are more composite than 
plants, 2. ¢., their organs are more complex and numerous, 
and more specially devoted to particular purposes. Repe- 
tition of similar parts is a characteristic of plants; of ani- 
mals, it is differentiation. Most animals, moreover, have 
fore-and-aft polarity ; in contrast, plants are up-and-down 
structures, though, in this respect, they are imitated by 
radiated animals, like the Star-fish. 
(5) Physiology—In their modes of nutrition, plants and 
animals stand widest apart. Yet “the difference” (says 
Owen) “is, at most, one of form and proportion of the in- 
ternal cavities, and of their external openings: they are 
the same as to function.” As a rule, plants absorb and 
assimilate food by the external surface, which is therefore 
greatly extended by leaves and roots. As a rule, animals 
receive nourishment through a mouth into an internal 
permanent or temporary stomach. But there are certain 
molds which seem to swallow their food,’ and Tape- 
worms have neither mouth nor stomach. 
A plant in the seed and an animal in the egg exist in 
similar conditions: in both cases a mass of organic matter 
accompanies the germ. When this supply of food is ex- 
hausted, both seek nourishment from without. But here 
