MORPHOLOGY OF ANIMALS 



the ectoderm surrounding the mouth is folded in at the 

 mouth opening, thus forming an ectodermal tube, or 

 stomodaeum, which opens at the inner end into the gastric 

 cavity. Among chordates this ectodermal invagination 

 forms only the mouth cavity, the oesophagus being derived 

 from the endoderm. In all Cnidaria, Ctenophora, and 

 Platoda there is but one opening into the gastric cavity, 

 the mouth, and through this single opening food is taken 

 in and undigested remnants cast out. In the Nemertinea, 

 and with a few exceptions in all higher animals, there 

 is a second opening into the gastric cavity, namely the 

 anus, through which the ejecta pass out. The anus is 

 formed by an infolding of the ectoderm which meets and 

 fuses with a portion of the gastric wall; this terminal 

 ectodermal portion of the digestive tract is the hind-gut, or 

 proctodaeum. 



Fore^ Mid, and Hind-Guts. With the formation of an 

 anus the digestive tract becomes tubular, with mouth at 

 one end and anus at the other, and the entire canal is 

 divisible into three portions, an ectodermal stomodaeum 

 or fore-gut, an endodermal mid-gut, and an ectodermal 

 hind-gut. The relative development of these three por- 

 tions differs much in difFerent phyla; for example among 

 chordates the fore-gut is limited to the mouth-cavity, and 

 the hind-gut to an insignificant terminal portion of the 

 intestine, while the mid-gut gives rise to all the inter- 

 vening portions of the digestive tract (Fig. 12, B). Among 

 arthropods, on the other hand, the mid-gut is limited to an 

 extremely small portion of the digestive tube between the 

 stomach and the intestine, while all the remaining portions 



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