SYSTEMATIC POSITION 



145 



Apart from the distinctive characters of the six " classes " into 

 which the Craniata are divided, two or three of these classes may 

 possess important structural features in common by which they 

 are distinguished from others. Thus, Cyclostomata, Fishes and 

 Amphibia agree with one another, and differ from all the remaining 

 groups in breathing by gills and in possessing lateral line sensory 

 organs during part, or the whole, of life. Their embryos have no 

 investing amnion, neither does the sac-like outgrowth from the 

 hind-gut, which is known as the allantois, if present at all, ever 

 extend beyond the coelom to form an embryonic investment or 

 to act as a primitive breathing organ. Hence, therefore, the 

 terms Ichthyopsida, Anamniota, and Anallantoidea have been 

 applied to these three classes. Similarly, the term Sauropsida, as 

 applied to Eeptiles and Birds, is a convenient means of giving 

 expression to the fact that, underlying the most striking diversity 

 of outward form and habits, there is a community of inward 

 structure which justifies the conclusion that these animals are 

 more closely related to one another than either group is to 

 any other class of Craniates. And again, the application 

 of the terms Agnathostomata and Gnathostomata brings into 

 sharp relief the fundamental distinction between the Cyclo- 

 stomata and all the remaining groups of Craniata which is 



VOL. VII L 



