DEVELOPMENT OF THE POLYPIDE. 43 



denied by Braem and Krabpelin, are probably to be traced back 

 to this layer.* 



It is thus evident that the inner layer of the bilaminar polypide- 

 rudiment yields the ectodermal epithelial layer of the jjolypide, and 

 also the epithelial lining of the enteric canal and those parts Avhich 

 we have been accustomed to attribute to tlie entoderm. From the 

 outer layer of the polypide-rudiment are derived the mesodermal 

 structures (the splanchnic layer of the mesoderm, the retractor- 

 muscles, the lining of the tentacle-cavities, etc.). 



When the rudiments of the organs described above have fully 

 developed, the atrial cavity of the polypide-rudiment becomes 

 connected witli the exterior, and through the aperture so formed 

 the anterior portion of the polypide with its crown of tentacles, 

 the introvert, can be protruded and extended. 



It must strike the reader as very remarkable that, according to the above 

 statements, tlie whole lining epithelium of the alimentary canal (both the part 

 usually derived from the ectoderm and the entodermal part) is derived from 

 one and the same rudiment, the inner layer of the sac-like polypide-rudiment. 

 In the primary zooecium of Bugula it was possible to trace this layer back 

 to an invagination of the ectoderm of the larva ; and in the same way, in the 

 polypide-rudiment of the buds and of the regenerating individuals it may be 

 traced back to the ectoderm of the zooecia. If these observations are correct, 

 we should be forced to assume that the whole enteric canal here originates from 

 the ectoderm. 



Several attempts have in consequence been made to find some other origin for 

 the middle (entodermal) part of the alimentary canal. A suggestion of a distinct 

 origin for the enteron is yielded by the constant connection discovered by 

 Repiachoff (No. 30), at later stages, between the enteric rudiment of the 

 polypide and the so-called hroivn body. In the primary zooecium, the brown 

 body contains the mass which has arisen by the degeneration of the larval 

 organs and of the central tissue, while the brown bodies found in the degen- 

 erated zooecia of the adult colony enclosed in cellular envelopes of their own 

 nmst be regarded as remains of the degenerated parent-polypide. The rudiment 

 of the intestine, whicli in the bud of the newly-forming polypide is originally 

 connected with the brown body through strands of funicular tissue, at a later 

 stage comes into contact with it, and is said finally to grow round it and to 

 receive it into the interior of the enteric cavity. The last remains of the brown 

 body are then said to be expelled through the anal aperture of the newly- 

 formed polypide. During this circumcrescence, according to Ostroumoff 

 (No. 26), the epithelial layer of the stomach-caecum is yielded by the cells 



* [Oka, Jour. Sci. Coll. Japan, Vol. viii. , p. 339, states that the Ecto- 

 proctous Polyzoa have no nephridia. He does not regard the ciliated portions 

 of the coelomic ei^ithelium, which ajjparently open by a pore under the median 

 tentacle, as nephridia. He states that the excretory function is carried on by 

 free mesodermal cells, which leave the body through the above-mentioned pore, 

 their passage to the exterior being facilitated by the presence of cilia on the 

 cells of the j)eritoneal epithelium adjacent to the pore. The tube-like character 

 of these modified peritoneal areas is due to the presence in the Phylactolaemata 

 of an epistome-lophophoral partition. — Ed.] 



