ORIGIN AND ST RUCTURE OF THE HEAD 23 



supposing that the two cerebral ganglia, which originate 

 separately, have not united by a commissure. They have 

 remained separate, the oesophageal nerve-ring as a conse- 

 quence not being closed anteriorly, and have passed, to- 

 gether with the oesophageal connectives which form tVe com- 

 munication with the ventral ganglion chain, into the optic 

 vesicles with their stalks, while the anterior ganglia of the 

 ventral chain give rise to the brain. He too, however, con- 

 sidered the Vertebrate mouth as a secondary formation and 

 found, with KUPFFER (1894), the palaeostoma again in the 

 common invagination from which in Cyclostomes both the 

 olfactory organ and the hypophysis take their origin. No 

 facts, however, can be adduced in favour of any of these 

 hypotheses and after their examination we can only con- 

 clude that MiNOT's statement that "il n'a ete propose 

 aucune hypothese ayant rapport a revolution de la tete 

 du Vertebre aux depens du type Ann^lide qui ne puisse 

 encourir des objections insurmontables" has not yet lost 

 its validity. 



All the above cited authors agree in that the Vertebrate 

 mouth is a secondary one, but why the old mouth should 

 have been lost and replaced by a new one, is not easily 

 explained from their theories. Gaskell (1908, p 55), who 

 derives the Vertebrates from Arthropodan ancestors in a 

 very adventurous way, lets the cerebral and infra-oesophageal 

 ganglia in the latter increase so much in size that the 

 oesophageal ring is reduced to a very narrow passage for 

 the gut, a process which finally resulted in a squeezing out 

 the latter by the increasing nerve-mass ("antagonism between 

 cephalization and alimentation"). The whole gut then passes 

 into the medullary tube of Vertebrates (cf. above, p. 11). 

 I hardly need point out once more how evident and natural an 

 explanation of such a curious phenomenon as the loss of a 

 mouth is afforded by my theory. But with reference also to 

 the problem of the formation of the Vertebrate head the theory 

 leads to most unexpected results, as I hope now to show. 



Praeoral lobe in Annelids. — In the development of the 

 Annelids the umbrella or episphere of the trochophora- 

 larva gives rise to the praeoral lobe or prostomium and to the 

 cerebral ganglia of the worm, the subumbrella or hyposphere 

 lengthens out into the segmented soma. It was Kleinenberg 

 (1886, p. 181) who first opposed these two regions of the 

 Annelids body to each other and distinguished them as 



