Vv SYSTEMATIC POSITION 145 
AGNATHOSTOMATA. 
r Without biting 
¢ 1. Cyctosromara. 
| Lampreys and Hag- 
Fishes. jaws. 
ICHTHYOPSIDA. 
Breathing by gills at some 
) 
\ 
{ 
J 
period of life. II. Piscxs. 
ANAMNIOTA. True Fishes. 
No embryonic covering | 
. 5 » 
eeaaninnt Ill. Aa HIBIA. 
Newts, Frogs, and 
ANALLANTOIDEA. Toads. 
No embryonic respiratory 
organ or allantois. \ GNATHOSTOMATA. 
( IV. Reprirta. With biting jaws. 
| Lizards, Snakes, 
| Turtles, and 
| 
| 
L 
| SAUROPSIDA. Crocodiles, 
AMNIOTA. 
Amnion present. V. AVES. 
4 Birds. 
ALLANTOIDEA. 
Allantois present. VI. MAMMALIA. 
l Hairy Quadrupeds. 
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 
apphed to Reptiles 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 
