tain cells
are budded off at an early stage of development from one or both of the
two original layers, to form later a third layer, the mesoderm, which
lies between the ectoderm and endoderm; such forms have therefore
received the name Triploblastica. At the same time it is necessary to
observe that it is by no means certain that the mesoderm found in
various groups of Metazoa is a similar or homologous formation in all
cases. A second essential difference between Coelentera and other
Metazoa (except Parazoa) is that in the former all spaces in the
interior of the body are referable to a single cavity of endodermal
origin, the "gastro-vascular cavity," often termed the coelenteron: the
spaces are always originally continuous with one another, and are in
almost every case permanently so. This single cavity and its lining
serve apparently for all those functions (digestion, excretion,
circulation and often reproduction) which in more complex organisms are
distributed among various cavities of independent and often very diverse
origin.

In the Coelentera the ectoderm and endoderm are set apart from one
another at a very early period in the life-history; generally either by
delamination or invagination, processes described in the article
EMBRYOLOGY. Between these two cell-layers a mesogloea (G. C. Bourne,
1887) is always intercalated as a secretion by one or both of them; this
is a gelatinoid, primitively structureless lamella, which in the first
instance serves merely as a basal support for the cells. In many cases,
as, for example, in the Medusae or jelly-fish, the mesogloea may be so
thick as to constitute the chief part of the body in bulk and weight.
The ectoderm rarely consists of more than one layer of cells: these are
divisible by structure and function into nervous, muscular and secretory
cells, supported by interstitial cells. The endoderm is generally also
an epithelium one cell in thickness, the cells being digestive,
secretory and sometimes muscular. Reproductive sexual cells may be found
in either of these two layers, according to the class and sub-class in
question. The mesogloea is in itself an inert non-cellular secretion,
but the immigration of muscular and other cells into its substance, from
both ectoderm and endoderm, gives it in many cases a strong resemblance
to the mesoderm of Triploblastica,--a resemblance which, while probably
superficial, may yet serve to indicate the path of evolution of the
me