xm PHYLUM CHORDATA 579 



tion. Chaetopods have been compared with inverted Vertebrates 

 the ventral nerve-cord being homologised with the neuron ; the 

 metameric muscular, circulatory, and excretory systems lend support 

 to this view. Elaborate comparisons have been instituted between 

 the brain of Cyclostomes and Fishes and those of Crustacea and 

 Xiphosura, and it has been sought to explain the neurocoele as the 

 discarded Arthropod enteric canal. But if Amphioxus and the 

 Urochorda, to say nothing of the Adelochorda, are branches from 

 some low part of the chordate stem a fact it seems hardly possible 

 to doubt it is obvious that there can be no direct connection 

 with the highly specialised classes referred to. If, for instance, the 

 lower Craniata sprang either from a Chsetopod-like or from a 

 Limulus-like ancestor, Amphioxus and the Tunicates must either 

 have no connection at all with Vertebrates, or must have undergone 

 a quite inconceivable amount of degeneration. 



As far as we can see, Amphioxus and the Urochorda show not the 

 slightest approach to any other phylum; Balanoglossus has cer- 

 tain affinities with Echinoderms ; Rhabdopleura and Cephalodiscus 

 are in some respects related to, and were formerly classed with, the 

 Polyzoa Ectoprocta ; Phoronis has been assigned both to the 

 Polyzoa and to the Gephyrea. On the whole it may safely be 

 said that the ancestry of Vertebrates is still an unsolved problem. 



With regard to the higher classes, Amphibians may be held to 

 have arisen from a Fish-type allied to the Dipnoi, the resemblances 

 of which to the Amphibia are so great as to lead some authors to 

 place them in a distinct class intermediate between Fishes and 

 Amphibia. The chief difficulty in this case and it is a serious 

 one is the derivation of the pentadactyle limb from the Fish's fin, 

 a transformation of which no satisfactory explanation is at present 

 offered either by anatomy, embryology, or palaeontology. 



Reptiles may be considered to have arisen from a generalised 

 amphibian stock, but there is no direct evidence on this point, 

 and, apart from purely theoretical considerations, there is nothing 

 to show how or why gills vanished so completely as to leave no 

 trace of their existence beyond the branchial clefts, or by what 

 steps the allantoic bladder became precociously enlarged into an 

 embryonic respiratory organ. The precise mode in which the 

 protecting amnion arose is also very doubtful, but from theoretical 

 considerations its development in the Hedgehog (p. 560) seems to 

 indicate a more primitive condition than obtains in the other 

 Mammalia or even in Sauropsida. 



Birds appear to be undoubtedly derived from true Reptiles, 

 although nothing is known of the actual ancestral form. In spite 

 of the enormous adaptive differences between the warm-blooded, 

 feathered, bipedal Bird, and the cold-blooded, scaly, quadrupedal 

 Reptile, the connection between the two is far closer than 

 between any other two vertebrate classes. 



p p 2 



