teristically in every leaf. Through the combined operation of this green color either singly or aided by the leaf-protoplasm—and the action of light, plants decompose the carbonic acid of the air, as every schoolboy knows, and, retaining the carbon to aid in the formation of
Fig. 18.—Hydræ. (In both figures young hydræ are represented budding from the side of the parent).
starch, set free the oxygen, which thus returns to the atmosphere, and is welcomed by the animal hosts. The hydra, or common fresh-water polyp (Fig. 18), many animalcules, and certain worms of a low type, possess this chlorophyl. Like dishonest manufacturers, they seem 
Fig. 19.—Rotifera. to have infringed the patent-rights of the plant to elaborate this green color. And it is no longer matter of theory, but ascertained fact, that these green animals are capable, like the plants, of absorbing carbonic acid—usually a fatal gas to the animal constitution—and of elaborating starch therefrom like their plant neighbors. Thus a simpler mode of feeding, obviating the necessities of animal existence in the way of digestive apparatus, has apparently led to the simplification of structure. Degeneration has followed in the worms just mentioned, as the result of their imitation and acquirement of vegetative powers of feeding; and it is probable that other alterations in the way of dietary, of less sweeping character than that just mentioned, will affect, in like retrogressive fashion, the animal constitution.
Some of the most curious cases of degeneration known to us illustrate the total disappearance of digestive apparatus even in some beings, in which, as in the stylops already mentioned, one sex becomes
