OTHER AXNULATA 



309 



Germ Layers and their Derivatives ic the Earthworm 



Fig. 149. — Germ layers of the earthworm and their derivatives. 



In the development of Metazoa, with the possible exception of the Porifera and Mesozoa, 

 there appear at an early stage of differentiation what are known as the germ layers. In the 

 Coelenterata, which are diploblastic animals (c/. p. 260), there are only two germ layers, the 

 ectoderm and the endoderm. In higher metazoa, a third germ layer, the mesoderm, 

 develops between the ectoderm and endoderm. Thus the gastrula of an earthworm (Fig. 

 148, G) is like a hydra with its ectoderm and endoderm surrounding the enteron, with the 

 addition of mesoderm, in which the coelome is developing. The Platyhelminthes (c/. Fig. 

 117, p. 240 and Fig. 118, p. 245) represent a condition intermediate between Ccelenterata 

 and Annulata to the extent that they possess ectoderm, mesoderm, and endoderm sur- 

 rounding the enteron, and are thus triploblastic, but are not coelomate animals, since they 

 do not possess a ccelome within the mesoderm. Although there are many exceptions, the 

 three germ layers may be identified in the development of the vast majority of metazoa, 

 and they normally give rise to certain adult parts that may be broadly homologized in the 

 several phyla (c/. Fig. 218, p. 417). 



Other Annulata 



Classification. — The Phj-lum Annulata includes four classes: 

 the Chcetopoda, which is the class to which the earthworms belong; 

 the Archi-annelida (Fig. 150 A), a small group of segmented worms 

 with either a very primitive or a somewhat degenerate organiza- 

 tion; the Sipunculoidea (Fig. 150 B), a group of marine worms that 

 are greatly modified from the fundamental segmented condition of 

 the group; and the Hirudinea, or leeches (Fig. 150 C), another very 

 specialized group in which the primary segmentation has become 

 obscured by subdivisions of the original metameres and other mod- 

 ifications related to the mode of fife, which is parasitic in many of 

 the species. 



