Practically this classification is not used, but we shall see that from our point of view it is a useful one. In the most general scheme the divisions are:—
1. Invertebrata, animals without backbones.
2. Vertebrata, animals with backbones.
The invertebrata are divided into sub-kingdoms, of which the protozoa form one. These protozoa possess, as it were, only negative properties. In their simplest form they are mere masses of protoplasm, even lacking an investing membrane or coat, and never, even in the highest forms, possessing distinct organs. It is this simplicity which at once separates them entirely from all other animals.
The other sub-kingdoms are:—
Cœlenterata, of which the jelly-fishes are a type; animals possessing an alimentary canal, fully communicating with the general cavity of the body, but without distinct circulatory or nervous systems.
Annuloida, of which the star-fishes are a type; animals having the alimentary canal shut off from the body-cavity, and possessing a nervous system, and in some a true circulatory system.
Annulosa, of which worms, lobsters, and insects are types; animals composed of definite segments, arranged serially, always possessing true circulatory and nervous systems.
Mollusca, of which oysters and whelks are types; animals which are soft-bodied, often bearing a shell, always possessing a distinct nervous system and mostly with a distinct heart.
In old systems of classification, the Cœlenterata and Annuloida were united into one sub-kingdom, the Radiata, in consequence of their radiate or star-like structures.