220 COMPARATIVE ZOOLOGY 



sea moss (Polyzoa), and corals, often resemble vegetable 

 forms, although in structure widely removed from 

 plants. Similar conditions of life are here accompanied 

 by an external likeness. In free-living animals this 

 resemblance is not found. 



(4) Structure. A plant is the multiplication of the 

 unit a cell with a cellulose wall. Some simple ani- 

 mals have a similar simple cellular structure ; and all 

 animal tissues, while forming, are cellular. But this 

 character, which is permanent in plants, is generally 

 transitory in animals. In the more highly organized 

 tissues the cells are so united as partly or wholly to lose 

 their individuality, and the characteristic part of the 

 tissue is the intercellular substance, while the cells 

 themselves are small and unimportant, or else the cells 

 are fused together and their dividing walls become in- 

 distinct, as in glandular tissue. Excepting the lowest 

 forms, animals are more composite than plants, i.e., 

 their organs are more complex and numerous, and 

 more specially devoted to particular purposes. Repe- 

 tition of similar parts is a characteristic of plants ; and 

 when found in animals, as the angleworm, is called vege- 

 tative repetition. Differentiation and specialization are 

 characteristic of animals. Most animals, moreover, 

 have fore-and-aft polarity ; in contrast, plants are up- 

 and-down structures, though in this respect they are 

 imitated by radiate animals, like the starfish. Plants 

 are continually receiving additional members ; most 

 animals soon become perfect. 



(5) Physiology. In their modes of nutrition, plants 

 and animals stand widest apart. 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 exhausted, both 

 seek nourishment from without. But here analogy 



