STRUCTURAL UNITS. 25 



didermic (or diploblastic). The large phylum of Ccelenterata 

 is didermic. The outer layer is known as ectoderm, the 

 inner as endoderm, and the space enclosed by them is the 

 archenteron. The space between the layers is usually filled 

 with a jelly-like substance called the mesoglcea. The third 

 type is the tridermic (or trip lob lastic). 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. 



/^y!y^'^ :^ ^^^^\ Entoderm. 



Mi /^^vvH^^^^X^^^^'^ uu ^Mesoderm. 



CH H-T\ Y^^^^"^ ttl ^Ectoderm. 



Note the somatic and splanchnic layers of mesoderm joined hy 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 entoderm 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 archicoele (cf. monoblastic types). In this archicoele 

 is arranged the third layer or element. It may consist (in 

 the Archicoela or Accelomata) 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- 

 helnmtthes) which end internally in blind tubes or sacs. In 

 the second type (the Ca^lomata) the greater part of the meso- 

 derm is formed into a definite epithelium limiting a cavity 



