HAECKEL'S AlsrTHROPOGENIE. 243 



separation of the intestinal tract into respiratory and digestive parts, 

 by the formation of an anus and a blood-vascular system, by the 

 ciliated external surface and the persistent hermaphroditism. The 

 nearest living representative is the peculiar -vrorm, balanoglossus. 



From this scolecida group spring the eighth stage, the chordonia. 

 In these a chorda was developed by the necessity of an axial line of 

 attachment for locomotive muscles, and in correlation with this there 

 took place the backward prolongation of the nervous centres. The 

 nearest living representative of the chordonium is the appendicularia 

 and the larval ascidian ; but these must rather be regarded as com- 

 mon descendants from the chordonia than as ancestral forms of the 

 vertebrata. 



In considering the phylogenesis of the vertebrata, it is thus seen 

 that we need pay no attention to any of the other great groups of 

 the animal kingdom — annelides, arthropods, echiaoderms, molluscs — 

 for these diverged in different directions from the lower ccelomati 

 before the scolecida were developed. 



The vertebrata sprung from the chordonia in the archozoic age, for 

 we find in upper silurian strata remains of a primordial selachian. 

 Palaeontology can tell us nothing of our oldest vertebrate ancestors, 

 but it is certain that all were derived from a skull-less form, from 

 which the living acrania and craniota have diverged. This consti- 

 tutes the ninth stage, and may be called provertebratum, of which 

 the only acraniate vertebrate living, "amphioxus," is the nearest 

 representative. 



The craniota have diverged in two lines, the living monorrhuia and 

 the amphirrhina : the former are represented solely by the lampreys 

 and ray fishes, have advanced little, and are almost extinct ; the 

 latter comprise all other vertebrata. The tenth stage would be one 

 of the primordial monorrhina, intermediate between the acrania and 

 the amphirrhina. 



Between the amphirrhina (gnathostomata) and the lowest verte- 

 brates there is a wide break, for besides the paired nose, there are 

 other essential distinguishing characteristics, such as the presence of 

 the maxillary and internal branchial arches, of the swim-bladder or 

 lung, and of the two-paired limbs, of the sympathetic nervovis 

 system, spleen and pancreas. 



The oldest group of the amphirrhina, from which all others diverged, 

 was that of the selachia. The ganoids and the dipnoi form two 



