STOMODAEUM AND MEDULLARY TUBE 



nervous system had existed and that the exact situation of 

 this passage was to be looked for in the fossa rhomboidea, 

 between the crura cerebelli. 



In this way only was it possible to homologize the 

 cerebral ganglion of Annelids with the brain of Vertebrates. 

 Rightly DOHRN (1875, p. 4) emphasizes the fact that the 

 mouth of Vertebrates is an organ which ontogenetically 

 appears only very late and therefore cannot be estimated as 

 of high phylogenetic antiquity. "Der Embryonalleib eines 

 Wirbelthiers ist fast vollstandig ausgebildet, alle grossen 

 Organsysteme bestehen bereits, die Circulation vollzieht 

 sich schon, — und noch immer besitzt der Embryo keine 

 Mundoffnung," while in all other groups the mouth is one 

 of the first organs to be formed. 



Other solutions have been proposed for the difficulty of 

 the different situation of the cerebral part of the nervous 

 system in Annelids and Vertebrates, a number of them being 

 mentioned at the beginning of the second chapter. One of 

 the most acceptable seems to me to be that suggested by 

 Hatschek which is mentioned on page 36. Yet there is one 

 drawback which it has in common with the others cited 

 above, viz: that the entirely different behaviour and fate of 

 the blastopore in Chordates and in Annelids are not accounted 

 for, and it is just the latter circumstance, which no doubt 

 has converted so many to the derivation of Vertebrates from 

 iorms related to the Enteropneusts. 



Protostomians and deuterostomians. — A great importance 

 has been attributed recently to the fate of the blastopore by 

 OROBBEN (1908) in his well-known classification of the 

 animal kingdom. By RAY LankesTER the sub-division of 

 the Coelomata was opposed to that of the Coelenterata, 

 both constituting together the subregnum Metazoa of 

 HAECKEL. By Hatschek (1888-1891) the Coelomata were 

 •subdivided into three phyla: the Zygoneura, the Ambulacralia 

 and the Chordonia. The foundation of the first group to 

 which the Platyhelminthes, Nemertinea, Nemathelminthes, 

 Kotifera, Brachiopoda. Bryozoa, Annelida, Mollusca and 

 Arthropoda belong, followed from HATSCHEK's trochop- 

 hora-theory, the trochophora and the protrochula larva 

 being the point of origin from which the different forms 

 "belonging to it have developed. In the second group it is 

 equally tiie pelagic larva which unites Echinodermata and 

 Enteropneusta, while the combination of Tunicates and 



