All the Enterozoa we have so far considered possess no cavity other than the archenteron, the gonads being masses of cells, derived ultimately from the endoderm, which lie in pockets in free communication with the primitive gut. The next stage in development involves the separation of these pockets from the gut, either by an actual pinching off or by a migration of the cells which will form the gonad into the mesoderm and their sub sequent arrangement into a vesicle. The independent cavity so formed is held to be the beginning of the coelom. This cavity, whatever its extent, is a morphological entity, independent of the enteron or true gut, and of the bloodspaces and of all other cavities, including that of the blastocoel, which is formed during early embryonic life by the separation of the first formed cells, or blastomeres. Part at least of the mesoderm arises from the walls of the coelom in all animals which possess that structure. (See EMBRYOLOGY.) The presence of a coelom, even if it be no more extensive than the cavity of a gonad, thus enables us to divide all En terozoa into two groups or grades, the Acoelomata and the Coelomata.
The establishment of the coelom, by isolating the gonads, and the increasing importance of the mesoderm, made the problem of the nutrition of the whole animal more difficult. In the Acoelo mata food transport was carried out by the development of canals along which food particles could be transported by ciliary cur rents throughout the body. This mechanism is replaced in all
higher animals by a blood vascular system, consisting of canals or spaces through which a fluid can be caused to travel to every part of the animal. This fluid, in its passage through the vessels which lie on the wall of the gut, becomes charged with food sub stances in solution, and is capable of giving them up to any organ which requires them.
By its mere presence as a circulating fluid which passes through the whole body and is not itself actively metabolic, the blood must serve as a carrier of oxygen, even though a poor one. Its respiratory function becomes far more important in higher ani mals with the introduction of respiratory pigments, carried by the blood, capable of forming easily dissociated compounds with oxygen.
The same factors which led to the introduction of a vascular system were responsible for the initiation of a definite excre tory system, at first in the form of a system of tubes, of ecto dermal origin, ending blindly in a hollow cell with a bunch of cilia lying in the cavity. Subsequently, when the coelom became an extensive cavity surrounding the gut, these primitive tubes, the nephridia, became connected with it, usually through the intervention of other structures, the coelomoducts, whose pri mary function was the transmission of the gametes to the exterior.
The classification which results from the application of these and other criteria is as follows :—