An animal is classified in accordance with its morphology, anatomy, histology, and embryology. Morphology determines its general shape, the position of its limbs, eyes, and mouth, and the covering of its body; anatomy, the arrangement of its internal organs, such as the position of its heart, lungs, stomach, etc.; histology, the character of the tissues of the body; and embryology, the method of the development of the animal from the embryo to maturity. It is only after these exact discriminations have been made that the groups are arranged. Owing to the greater accuracy resulting from histology and embryology (methods which have been employed only in later years), many changes in classification have been made, and as science advances will continue to be made.
The primary groups are based on broad general characteristics, but their divisions and subdivisions are determined by closer distinctions. Animals having shells differ from those having a cartilaginous or those having a crustaceous covering, and are placed in different groups. Yet mollusks having a single or a double shell, having spiral or flat forms, living on land, in fresh water, or in the sea, while differing from one another, are all of one group. Lobsters and crabs, although both have crustaceous coverings, are very unlike; and again, there are many species of both lobsters and crabs.
To group individuals, noting resemblances as well as differences, a system of classification has been arranged with the following divisions:
Kingdom, Phylum, Class, Order, Family (or Suborder), Genus, Species. [pg021]
The biological division, or discrimination, between animal and vegetable life, is based on the manner of assimilating food. Plants feed upon mineral substances, or, in other words, assimilate inorganic matter, while animal life requires for its support vegetable or some other organic matter.
Animal as well as vegetable life in its lowest forms begins with one-celled organisms, which are called respectively /Protozoa/ (first animals) and /Protophyta/ (first plants). Both of these divisions are composed mostly of microscopic objects, and, together with other minute forms of life of the marine species, constitute a great part of the /plankton/, or free-floating organisms of the sea. These minute organisms seem like connecting-links between the two kingdoms. They were claimed by both botanists and zoölogists until the use of the microscope made close observation of minute structure possible.