96 THE ROYAL SOCIETY OF CANADA 



tives on the one hand, and through the pharynx and its derivatives 

 on the other, the maximum importance is assigned at the earhest 

 moment to the respiratory processes in evolution. It may seem 

 singular that such an apparently obvious point has not been brought 

 out in previous schemes. To some extent it has been implied, though 

 not with the precision of a decisive cleavage. John Hunter (1728- 

 1793) attempted to set up a classification, known as Hunter's Cardiac 

 System, based upon the characters of the heart and respiratory organs. 

 Mammals and Birds with their four-chambered heart were Tetracœ- 

 lia; Reptiles and Amphibians, with three-chambered heart, were 

 Tricoelia; Fishes with two-chambered heart (undivided atrium and 

 undivided ventricle) were Dicœlia. Hunter had detected the 

 existence of two auricles with one ventricle in the bivalve Mollusca, 

 but as Owen remarks in the preface to Hunter's Animal Oeconomy 

 (edition of 1837): "His perception of the physiological relations of 

 these different cavities prevented him from associating the mussel 

 with the tortoise on account of this tricœlous structure of the heart." 

 The two auricles of the mussel's heart perform the same function of 

 returning oxygenated blood from the gills to the ventricle; the auricles 

 of the reptile's heart perform different functions; all play a vital role 

 in promoting the circulation of oxygenated blood. 



Huxley's three provinces of Vertebrata: Ichthyopsida, Saurop- 

 sida and Mammalia, were based primarily upon the respiratory 

 apparatus, that is to say, upon the presence or absence of functional 

 branchiae. The progress of palaeontology has tended rather to ac- 

 centuate the breach between fins and pentadactyle limbs, which 

 Huxley's system ignored, his Ichthyopsida embracing both fishes and 

 batrachians. Accordingly I venture to suggest a modification in 

 the extension of this term; but under the altered application it retains 

 a fundamental part of its original meaning in so far that Huxley's 

 Ichthyopsida, then as now, included Amphioxus, in which the persis- 

 tent notochord extends beyond the cerebro-spinal axis to the anterior 

 end of the body, whereas in all other Vertebrates the notochord stops 

 short behind the pituitary gland. Instead of ranging amphibians 

 alongside fishes, the course that seems to be indicated at the present 

 juncture is to employ Ichthyopsida as a middle term in a series, 

 placing Amphibia under Tetrapoda. In order to establish this propo- 

 sition a long discussion would be necessary, were it not that the need 

 has been met to a large extent in an essay by W. K. Gregory (1915). 

 The continued use of Ichthyopsida in Huxley's sense has also been 

 criticized by Gadow. In the new sense it may be contrasted with 

 Helminthopsida (Enteropneusta), thus bringing into relief the radical 

 cleavage between soft-bodied worms and firm-bodied chordates. 



