124 OUTLINES OF EVOLUTIONARY BIOLOGY 



and the gonads (Fig. 62, A, yon.) are accordingly situated close 

 to the surface of the body and no special ducts are required for 

 the conveyance of ova or spermatozoa to the exterior. 



In the great group Coelomata, which includes practically all 

 animals higher in the scale of organization than the ccelenterates, 

 we find a different state of affairs. The mesoglcea has been 

 replaced by a true cellular mesoderm (Fig. 62, B, mcs.), formed of 

 cells derived from ectoderm or endoderm, or from both, and in 

 the thickness of this layer a cavity is developed, the ccelom or 



end, 



FIG. 62. Comparison of Ccelenterate and Invertebrate Coaloinate Types of 

 Structure. 



A. Diagram of a transverse section of a ccelenterate animal. 



B. Diagram of a transverse section of an invertebrate coRlomate animal. 



coel. coalom or body cavity; ccel. ep. ccelomic epithelium lining body cavity; d.v. dorsal 

 blood-vessel ; ect. ectoderm ; end. endoderm ; ent. enteron or digestive cavity ; 

 g. a. genital aperture ; g.cl. genital duct (gonoduct) ; gon. gonad ; mes. mesoderm ; 

 mesgl. mesoglcea ; n.c. double nerve cord ; som. somatopleure or body wall ; spl. 

 splanchnopleure or gut wall ; v.v. ventral blood-vessel. 



body cavity (coel.}, which more or less completely surrounds the 

 digestive tube or alimentary canal (compare the development of 

 Amphioxus, Fig. 13, XI XIII) . The outer layer of the mesoderm 

 unites with the ectoderm to form the body wall or somatopleure 

 (som.) , while the inner layer unites with the endoderm to form the 

 gut wall or splanchnopleure (spZ.), and the body thus acquires the 

 form of a double tube. The body cavity is lined by a layer of 

 epithelial cells (coel. ep.), and it is from this coelomic epithelium 

 that the germ cells arise. The gonads (Fig. 62, B, gon.) therefore 

 project into the body cavity and into this cavity the mature germ 

 cells are primarily discharged. In the great majority of cases 



