XXIV PEOCEEDINGS 01" THE 



originating in a single formative cell, quite independently of the 

 nucleus ; the formative cell may, in this respect, be compared with 

 the entire body of the Infusorium. 



It is the endoplasm, or internal parenchyma of the Infusoria, that 

 has given rise to the most important differences of opinion ; and in 

 his account of this part of the Infusorian organism, Haeckel chiefly 

 directs his criticism against the views advocated by Claparede and 

 Lachmann and by GreefF. 



These authors, as we have already seen, compare the Infusoria 

 with tlie Coelenterata, and regard the endoplasm, not as a real part 

 of the body, but merely as the contents of the alimentary canal — as 

 a sort of food-mash or chyme contained in a spacious digestive cavity, 

 whose walls are at the same time stomach-wall and body-wall, and 

 into which the mouth leads by a short gullet. As Haeckel urges, 

 however, it needs only a correct conception of the intestinal cavity 

 throughout the animal kingdom, and of its distinction from the 

 body-cavity, in order to show the xmtenableness of this position. 

 The main point of such a conception lies in the fact that the intes- 

 tinal cavity and all extensions of it (gastrovascular canals &c.) are 

 always originally clothed by the endoderm, or inner leaflet of the 

 blastoderm, while the body-cavity is always found on the external 

 side of the endoderm, and between this and the ectoderm, or outer 

 leaflet of the blastoderm. The body-cavity and intestinal cavity of 

 animals are thus essentially different; they never communicate with 

 one another, and always arise in quite different ways. 



Again, the contents of a true intestinal cavity consist only of nu- 

 tritious matter and water — in other words, of chyme ; while the 

 fluid which fills the body-cavity is never chyme, but is always a 

 liquid which has transuded through the intestinal wall, and which 

 may be called chyle, or blood in the wider sense of the word. 



Haeckel has thus taken, I believe, the true view of the intestinal 

 and body -cavities of animals. He had already advocated it in his 

 work on the Calcareous Sponges. It necessarily involves a belief in 

 the homological identity of organization between very distant groups 

 of the animal kingdom — a belief which all recent embryological 

 research has only tended to confirm. 



It follows from this view that the cavity of the Coelenterata 

 would represent an intestinal cavity only, while a true body-cavity 

 would be here entirely absent. This way of regarding the cavity of 

 the Coelenterata is at variance with the conclusions of most other 

 axiatomists, who regard the coclenterate cavity as representing a 



