CHAPTER II. 



PHYLUM ENTEROPNEUSTA. 



Unsegmented Ghordata with a tripartite division of the body 

 and coelom, a dorsal preoral lobe, and a notochord-like structure 

 which is confined to the anterior (proboscis) region of the body. 

 Pharyngeal branchial apertures are present in all except Rhab- 

 popleura. 



The Enteropneusta are Chordates * which present the fol- 

 lowing important features. The body is divided into three 

 regions, the proboscis which is a dorsal preoral lobe, the collar, 

 and the trunk. In the anterior part of the trunk paired lateral 

 apertures : the gill-slits are present putting the alimentaiy 

 canal in communication with the exterior (except in Rhabdo- 

 pleura). The coelom, in the cases in which its development is 

 known, arises as five diverticula of the embryonic enteron, 

 viz., one unpaired pouch, which extends into the proboscis and 

 is called the proboscis cavity ; one pair of pouches which occupy 

 the collar-region and are known as the collar-cavities ; and 

 finally a posterior pair which occupies the whole of the trunk 

 region and constitutes the trunk-cavities. The proboscis cavity 

 opens to the exterior by a single or double pore ; the collar 

 cavities each by a pore ; while the trunk-cavities are devoid 

 of an external opening. 



The anterior part of the alimentary canal sends into the base 

 of the proboscis an unpaired diverticulum which has a charac- 

 teristic structure and has been identified as a notochord. The 

 central nervous system lies in the ectoderm, and there is a special 

 concentration of it in the dorso-median line of the collar. This 

 concentration comes, in the majority of forms, to lie in the 

 wall of a canal which is open at both ends. There is always a 



* See vol. 2. ch. i. 



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