154 GENERAL ZOOLOGY 



archenteron (?/), or primitive digestive cavity, which is 

 lined by the inner epithelium or entoderm (en). The 

 ectoderm and the entoderm are the primary germ layers, 

 and the metazoan is now a gastrula (G). 



Some animals (Fig. 68) are adults in the gastrula stage 

 and their development goes no farther. They remain in a 

 diploblastic, or two-layered, condition all their lives and 

 the archenteron serves their simple digestive needs. Most 

 metazoans, however, have a third layer, the mesoderm, 

 formed between the ectoderm and entoderm, and also have 

 modifications of the archenteron so that, instead of having 

 only one opening for the ingestion and rejection of food, 

 a mouth and anus develop. During the gastrula stage, 

 or soon after, metazoans take on the distinctive character- 

 istics of the phylum to which they belong. A coelenterate 

 remains a gastrula but forms tentacles, stinging cells, and 

 other structures proper for its adult life. Most flatworms 

 develop a great mass of mesodermal cells but retain the 

 original opening into the archeneteron for feeding. More 

 highly specialized animals develop far beyond the gastrula 

 stage; the three germ layers becoming greatly modified 

 by thickenings, the formation of cavities, foldings, and in 

 other ways. 



5. The Formation of the Coelom, or Body-cavity. After 

 passing through the gastrula stage a coslomate animal de- 

 velops a tubular digestive system, the original entrance 

 into the archenteron serving as one opening and a new 

 aperture forming by breaking through at the opposite 

 end (Fig. 64, H). Although there is some variation in 

 the way the coelom arises in different groups of animals, the 

 following description will serve to illustrate the general 

 method. Pairs of little pockets form along the sides of the 

 alimentary canal and push out against the body wall. 

 The cavities in these later lose their connection with the 

 alimentary canal (taking the form of closed sacs), then 

 enlarge greatly so as to envelop all the internal organs 

 (J), and fuse together (K). Thus a large space, the coelom 



