191 
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 vollständig ausgebildet, alle grossen 
Organsysteme ‘bestehen bereits, die Circulation vollzieht 
sich schon, — und noch immer besitzt der Embryo keine 
Mundötffnung,” 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 222. 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 
forms related to the Enteropneusts. 
Protostomians and deuterostomians. — A great importance 
has been attributed recently to the fate of the blastopore by 
GROBBEN (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 o 
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, 
Rotifera, 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 the pelagic larva which unites Echinodermata and 
Enteropneusta, while the combination of Tunicates and - 
