82 



INVERTEBBATA 



CHAP. 



stem 



to be of endodermal origin. It is distinctly stated that in Alcyonaria 

 the two dorsal ciliated filaments do not take part in the process of 

 digestion, and if they are of ectodermal origin this is also what one would 

 expect. But if we accept Appellofs account of the development of 

 the filaments in Zoantharia, we must suppose that the ectoderm in 

 these animals has acquired digestive functions. Since in all groups 

 of the animal kingdom which have been carefully examined, the 



distinction between pro- 

 tective ectoderm and 

 digestive endoderm is 



O 



the first physiological 

 differentiation to be 

 established, and the most 

 deeply rooted, one would 

 imagine it is unlikely 

 that Zoantharia should 

 in this respect form a 

 solitary exception. 



But some observa- 

 tions by Miss Pratt on 

 the digestion of Alcyon- 

 aria (1905), appear to us 

 to place the whole matter 

 in a new light. She 

 finds that gland cells 

 similar to those found 

 in the six ventral diges- 

 tive filaments are found 

 also in the stomodaeum, 

 and that the tissues in 

 these filaments are so 

 similar to the stomodeal 

 tissue that she believes 

 that in Alcyonaria as in 

 Zoantharia all the fila- 

 ments originate from the 

 ectoderm. The effect of the digestive juice is not to dissolve the 

 bodies of the prey, but to break them up into granules, which are 

 then ingested whole by the endoderm cells covering the mesentery 

 beneath the filament. The cells forming the filament do not ingest 

 anything. It thus appears that this so-called ectodermal digestion 

 is merely a preparatory process similar to that exercised by salivary 

 glands in Mammalia, which, like the filaments, are of ectodermal 

 origin, and that in all cases the final digestion and the assimilation 

 are functions performed by the endoderm alone. 



The muscular thickenings which run along the faces of the 

 mesenteries appear about the same time as the tentacles. The 

 supporting lamella becomes beset with branches on one side which 



%;-; 



>r:>~-^e$^W>"-<\irWFW& 

 ' 'i.-v.V' ', '.ii^"^ 



FIG. 62. Longitudinal section through the larva of 

 Ayaricia agar idles to show the ectodermal origin of 

 the mesenterial filament. (After Duerden.) 



coe, coelenteron ; end, endoderm ; fil, mR^enterial filament ; 

 nerv, nervous tissue at the aboral end of larva ; nem, nernato- 

 cyst. 



