STRUCTURAL UNITS. 25 
didermic (or diploblastic). The large phylum of Cwlenterata 
is didermic. The outer layer is known as ectoderm, the 
inner as exdoderm, and the space enclosed by them is the 
archenteron. The space between the layers is usually filled 
with a jelly-like substance called the mesoglea. The third 
type is the ¢ridermic (or triploblastic). In it there can be 
discerned, at least in early stages, three distinct primary 
layers or epithelia. Nearly all the higher animals are 
tridermic. In tridermic forms, the outer and inner layers 
are called the ectoderm and entoderm, whilst the middle 
Fig. 3.—DIAGRAM OF A TRIDERMIC ORGANISM, SEEN IN 
CROSS SECTION. 
Entoderm. 
Mesoderm. 
Ectoderm. 
Note the somatic and splanchnic layers of mesoderm joined by dorsal and 
ventral mesenteries ; the haemoccele is not seen as the mesoderm 
is closely adherent to the other layers. 
layer is the mesoderm. The space in the exfoderm is now 
called the mesenteron. The ectoderm and entoderm are not 
in contiguity, but there is always a more or less spacious 
body-cavity or cavity of the body, which is the primary body- 
cavity or avchicele (cf. monoblastic types). In this archiccele 
is arranged the third layer or element. It may consist (in 
the Archicela or Acelomata) of a mass of connective tissue, 
muscle-cells and gonads formed of more or less isolated 
cells (cystic) or layers (dermic). In this type the excretory 
organs are of the type called flame-cell tubules (see Platy- 
helminthes) which end internally in blind tubes or sacs. In 
the second type (the Ca/omaza) the greater part of the meso- 
derm is formed into a definite epithelium limiting a cavity 
4 
