In the direct development Bateson showed that the three divisions of the coelom arise as pouches constricted off from the archenteron or primitive gut, thus resembling the development of the mesoblastic somites of Amphioxus.
Ray Lankester to the members of a series of tubes, proved in some cases to be excretory in nature, which exist typically to the number of a single pair in most of the segments of the Chaetopod body, and open each by a ciliated orifice into the coelom on the one hand, and by a pore on to the exterior of the body on the other.
We may, with Sedgwick, suppose the coelom to have originated by the enlargement and separation of pouches that pressed outwards from the archenteron into the thickened body-wall (such structures as the genital pouches of some Coelentera, not yet shut off from the rest of the cavity), and they would probably have been four in number and radially disposed about the central cavity.
In Mollusca the coelom is reduced and consists of two parts, the pericardial cavity which surrounds the heart, and the cavity of the gonads or generative organs.
The chief difference is that the gonad or generative portion of the coelom is single and median, opening into the pericardium by a single posterior aperture.
It is therefore by no means certain that so profound a difference embryologically can be asserted to exist between the excretory nephridia and the ducts leading from the coelom to the exterior, which are usually associated with the extrusion of the genital products among the Chaetopoda.
One of the most formidable difficulties in the way of the attempt to reduce Actinotrocha to the Pterobranchiate type of structure is the condition of the coelom in the former.
Its cavity also is at first independent of the coelom though later invaded by the latter.
To this pericardial coelom is frequently added a gonocoel enclosing the gonads and the funnels of their ducts.
Of these latter, two grades were further distinguished by Lankester - those which remain possessed of a single archenteric cavity and of two primary cell-layers (the Coelentera or Diploblastica), and those which by nipping off the archenteron give rise to two cavities, the coelom or body-cavity and the metenteron or gut (Coelomata or Triploblastica).