102 THE ROYAL SOCIETY OF CANADA 



Urochorda (Tunicates) and Cephalochorda (Amphioxus), terms which 

 were introduced by Lankester in 1877 and adopted by F. M. Balfour 

 in his Comparative Embryology. 



The grades of organization are represented as the steps of com- 

 parative anatomy; they are determined by the characters of the 

 "^organs. Convergence is the condition where an organ, developing 

 along a particular line of descent, tends towards the level of that 

 attained along a parallel line. In the diagram it is expressed by the 

 parallel lines of descent tending to meet the horizontal lines of grada- 

 tion in respect of particular organs. For example, the gizzard of 

 certain fishes attains a high level of development along the piscine 

 line, far surpassing that of the average fish stomach and approaching 

 that of birds. If the oblique line of descent of fishes and the hori- 

 zontal grade of birds be produced, they will eventually meet at the 

 cross-roads of gizzard-convergence, while the lines of genetic descent 

 remain parallel and remote. Convergence is, therefore, found where- 

 ever one line of descent tends to encroach upon another average grade 

 of organization belonging to a different line. 



As a typical example of remote parallelism we may take the case 

 of Mollusca and Vertebrata, comparing them as regards the ingestion 

 of food. 



MOLLUSCA. VERTEBRATA. FOOD. 



Scolecomorpha Helminthopsida Scolecine ingestion 



(Solenogastres) (Enteropneusta) 



Acephala Acrania Ciliary ingestion 



Gastropoda Cyclostomi Suctorial ingestion 



Cephalopoda Gnathostonii Raptorial ingestion. 



References to Literature. 

 Boas, J. E. V. 



1882. Uber den Conus arteriosus und die Arterienbogen der Amphibien. 

 Morph. Jahrb. VIL, pp. 488-572. 



BUTSCHLI, O. 



1910. Vorlesungeu uber vergleichende Anatomie. Lief. 1. Leipzig, 

 see p. 41. 



Eycleshymer, a. C. 



1906. The Habits of Necturus maculosus. Amer. Nat. 40, pp. 123-136. 



Eycleshymer, A. C. 



1906. The Growth and Regeneration of the Gills in the Young Necturus. 

 Biol. Bull. X., pp. 171-175. 



Gadow, H. 



1909. Amphibia and Reptiles. Cambridge Nat, Hist. VIII. , see p. 5. 



