PREVIOUS INTERPRETATIONS OF CNIDARIA 173 



has become a general and unexceptional characteristic of Cni- 

 daria. The polymerization did not remain limited to the ten- 

 tacles only, it was also extended to the intestinal diverticula. 

 The only change which took place in these intestinal diverti- 

 cula of tentacles was a secondary obliteration of the lumen of 

 these diverticula; as a consequence of this change the axis of 

 tentacles became solid and the function of the diverticulum 

 changed. It has now^ been developed into a supporting organ 

 whose function has become more mechanical. This develop- 

 ment has not occured, as it seems, even in single cases in the 

 Anthozoa: their tentacles are always hollow, i.e. the lumen 

 of the diverticula of these tentacles has been preserved as a part 

 of the so-called gastrovascular system. It is only in Scyphozoa, 

 and even more so in Hydrozoa, that the cavities of the intes- 

 tinal diverticula of tentacles have become obliterated. 



In this connection, I cannot silently pass the fact that this 

 morpho -physiological specialization of the intestinal excres- 

 cences, which were originally hollow in order to make possible 

 the distribution of the fluid food, into solid organs with a pure- 

 ly mechanical function can be repeatedly observed in the 

 phylogeny of Metazoa (Fig. 33). Here I must limit myself 

 to a few remarks only. Even among the Cnidaria we can find, 

 besides the solid entodermal axes of tentacles, a case inTubula- 

 ridae, among Hydrozoa, where a cushion in the form of a circlet 

 and consisting of vacuoUzed entodermal cells had been deve- 

 loped around the stomach. Other cases have been described in 

 connection with the Turbellaria. I wdsh particularly to mention 

 here the so-called Hemichordata (Enteropneusta and Pterobranchia) 

 with their "bucal diverticulm," and finally the chorda dorsal is 

 which has given its name to the whole highest phylum, 

 to Chordata. These are certainly not homologous forms in the 

 strict sense of the classical comparative anatomy; they are, 

 nevertheless, parallel organs. We have here a morphogenetic 

 inclination of the intestinal tube locally to take over functions 

 which are not connected with the digestion; this has finally 

 led to the formation of new organs. 



