CLASSIFICATION OF ANIMALS. 7 



Thus, after reviewing the different characters by which it has been 

 attempted to distinguish the special subjects of the botanist and 

 zoologist, we find that neither sensation and motion, the internal 

 assimilating cavity, the respiratory products, the chemical constitu- 

 tion of the tissues, nor the source of nutriment, absolutely and un- 

 equivocally define the boundary between the animal and vegetable 

 kingdoms. We can only recognise the plant or animal when a cer- 

 tain number of their supposed characteristics are combined together. 



A rooted organism, exhaling oxygen, with tissues chiefly com- 

 posed of cellulose or of binary or ternary compounds, is, without 

 question, to be called a "plant." An irritable or locomotive orga- 

 nism, with a mouth and stomach, with gelatinous or albuminous 

 tissues, or chiefly composed of quaternary compounds, exhaling car- 

 bonic acid, is as certainly an "animal." But such "plants" and 

 "animals" are specially defined members of one and the same great 

 family of organised beings. 



An internal assimilative cavity, whether in the form of cells, canals, 

 or bags, is essential to all. The movement of a part, when stimulated, 

 is a property continued from the higher organised forms far down 

 into those that manifest the combined characteristics of the vegetable 

 kingdom, as, e. g. in the Mimosa pudica and the Dionma muscipula, 

 as to which the statement that such movements were destined solely 

 for the furtherance of formative operations, would be purely gra- 

 tuitous. Locomotion also crosses the supposed line, and is an en- 

 dowment of the embryos or spores of the sea-weeds. On the other 

 hand, the rooting or fixation of the organism is continued upwards 

 from the vegetable kingdom into the Radiate, the Articulate, and 

 the Molluscous divisions of the animal kingdom. The cellular and 

 cellulo-vascular forms of the assimilative cavity, common to plants 

 and sponges, is repeated in the cestoid entozoa, the astomous poly- 

 gastria, the rhizopoda, and in the early embryos of all higher animals. 

 Tissues of binary compounds are found in polygastria and ascidise, 

 and the chitinous coverings of insects and crustaceans have a much 

 closer resemblance to ligneous fibre than to proper animal tissues. 

 The presence of starch is of itself quite inadequate as a ground of 

 distinction, even were it proved to form part of the proper cell- 

 walls. And, on the other hand, nitrogen combines with carbon and 

 hydrogen to constitute the chief tissues of the sponges, algae, and 

 fungi. These exhale carbonic acid like well-organised animals, and 

 the polygastria exhale oxygen like typical plants. 



Thus the groups of characters that are essential to the true defini- 

 tion of a plant and an animal interdigitate, so to speak, in that low 

 department of the organic world from which the two great branches 



B 4 



