PREVIOUS INTERPRETATIONS OF CNIDARIA 179 



symmetric distribution, out of another extreme case, e.g. out 

 of a polyclad turbellarian, whose body is maximally flattened 

 and extended with a possibly large number of diverticula or 

 which can even form a net of digestive canals we would 

 have to accept, first, that diverticula had grown additionally 

 in other transversal directions and not only on the right and 

 left sides, and, secondly, that diverticula grew together which 

 appeared successively in lines parallel to the intestine, and 

 that this has finally led to the formation of longitudinal sarco- 

 septa. The first possibility is not only probable; it actually 

 takes place even if to slight degree and only in such species 

 which are not flattened but show^ instead in their transverse 

 sections the form of a square, and which possess four or 

 even more (instead of two) longitudinal series of intestinal 

 diverticula (e.g. the polyclad species Thysanoplana indica Plehn 

 and Pseudocerops periphaeus Bock). The second possibility is 

 theoretically probable, but it has not been supported by 

 observations and we will therefore prefer not to take this pos- 

 sibility into consideration. 



We will rather consider it more probable that in Anthozoa, 

 the formation of sarcosepta had been newly developed by 

 them; yet this formation goes back to a general tendency 

 of the intestine to form excrescences, diverticula, and folds. 

 This tendency had been inherited by the Cnidaria from 

 their turbellarian ancestors and it has finally led to the 

 formation of the gastrovascular system. The formation of lon- 

 gitudinal folds agrees better than the formation of diverticula 

 (which has been preserved in the development of the circlet 

 of tentacles!) with the transition to the polyp form which 

 stands erect and whose size has been constantly increased. It 

 should be mentioned here that the progressive development 

 of subtypes of Actiniaria led to the formation of species which 

 have remained soHtary and w^hose size can be considered as 

 comparatively very large (up to 1 m in diameter). Sarcosepta 

 have evolved both quantitatively (under the influence of poly- 

 merization) as w^ell as qualitatively (differentiation) in those 



