oelomic canals communicating
with a ventral canal by means of branchial canals which run down the
outer side of the primary gill bars. Into the dorsal canals the
nephridia open. In the intestinal region the coelom is only present on
the left side.

In the higher vertebrates (_Craniata_) the coelom is developed by a
splitting of the mesoderm into two layers, and a pericardium is
constricted off from the general cavity. In all cases the ova burst into
the coelom before making their way to the exterior, and in some cases,
_e.g._ amphioxus, lamprey (Cyclostomata), eels and mud-fish (Dipnoi),
the sperm cells do so too. The Cyclostomata have a pair of _genital
pores_ which lead from the coelom into the urino-genital sinus, and so
to the exterior.

In the Elasmobranch fish there is a _pericardio-peritoneal canal_
forming a communication between these two parts of the coelom; also a
large common opening for the two oviducts in the region of the liver,
and two openings, called _abdominal pores_, on to the surface close to
the cloacal aperture. In the Teleostomi (Teleostean and Ganoid fish)
abdominal pores are rare, but in most Teleostei (bony fish) the ova pass
directly down oviducts, as they do in Arthropods, without entering the
peritoneal cavity; there is little doubt, however, that these oviducts
are originally coelomic in origin. In the Dipnoi (mud-fish) abdominal
pores are found, and probably serve as a passage for the sperm cells,
since there are no vasa deferentia. In fishes a complete dorsal
mesentery is seldom found in the adult; in many cases it only remains as
a tube surrounding the vessels passing to the alimentary canal.

In the Amphibia, Reptilia and Aves, one cavity acts as pleura and
peritoneum, though in the latter the lungs are not completely surrounded
by a serous membrane. In many lizards the comparatively straight
intestine, with its continuous dorsal mesentery and ventral mesentery in
the anterior part of the abdomen, is very like a stage in the
development of the human and other mammalian embryos. In the mammalia
the diaphragm is complete (see DIAPHRAGM) and divides the
pleuro-peritoneal cavity into its two constituent parts. In the lower
mammals the derivatives of the original dorsal mesentery do not undergo
as much fusion and obliteration as they do in adult man; the ascending
and descending mesocolon is retained, and the transverse mesocolon
contracts no adhesion to the great omentum. It is a common