THE NEW GENEALOGICAL TREE 385 



Structures. A new problem which emerges here is what should 

 be understood as a coelom? This problem has already been 

 sufficiently discussed in the present study. The name itself 

 has been given to widely different body cavities which all 

 appear in the middle body layer while at the same time they 

 are of different origins. All these differences, however, can 

 be reduced in the last line to two categories which are divided 

 into several subcategories. We must distinguish between the 

 coeloms in a wider sense of the word which are all the body 

 cavities that are surrounded by the "mesodermal" cells that 

 form their epithelia, and the coeloms in a narrower sense 

 of the word, i.e. the body cavities which are developed around 

 the intestine and which possess their own epithelium. The 

 coelom is subject (which is very important!) to segmentation 

 during a certain phase of the phylogenetic development. 

 Whenever we speak of a coelom we usually think of this 

 coelom s.str. for which I have suggested the word peri- 

 gastrocoele to be used. This perigastrocoele played the most 

 important role in the phylogeny of the Eumetazoa and it is 

 therefore quite right if we take into consideration the develop- 

 ment of this perigastrocoel when we propose larger taxons. 

 Just in the point of this coelom s. I. we can observe cleanly 

 the great difference which exists between the two opposing 

 theories about the origin of the Eumetazoa. Those who accept 

 the colonial theory consider that all the body cavities of the 

 Metazoa, the intestinal cavity included (in the last line this 

 could also be considered as a coelom J*./.), are new formations 

 which had evolved completely independently from each other 

 and not all at the same time. The perigastrocoele, and this 

 is mainly the coelom to be discussed, is usually derived 

 from some sack-like outgrowths of the intestine, and the 

 coelom cavity from the intestinal cavity. Such a type of 

 development which can be observed directly and quite 

 frequently during ontogenies would presuppose that a 

 fundamental change takes place in the function of the sack- 

 like outgrowths of the intestine. 



