4 INTRODUCTION. 
The heart consists of two successive chambers, an auricle (atrium) 
and a ventricle, and in forms which respire by means of gills, contains 
only venous blood. With aerial respiration both chambers may become 
divided into arterial and venous halves. A dorsal aorta, lying above 
the alimentary canal, is always present. 
The sexes are usually separate. The reproductive and excretory 
systems are closely related, giving rise to a urogenital system. ‘The 
excretory ducts usually carry off the reproductive products (eggs and 
sperm). The urogenital ducts empty near the anus. Reproduction 
is strictly sexual; parthenogenesis and reproduction by budding do not 
occur and alternation of generations is unknown. The viscera are 
enclosed in a large body cavity (ccelom) which in the adult does not 
extend into the head. Each viscus is supported by a fold (mesentery) 
of the lining membrane of the cavity. 
For details of the classification of vertebrates reference must be 
made to special text-books of zoology, but as some of the larger groups 
must be referred to frequently, so these with a slight definition and one 
or two examples are given here. 
Serres I. CYCLOSTOMATA. 
These are eel-like in form, breathe by gills, have but one nostril, 
a circular mouth, incapable of closing, for no jaws are present. 
The skeleton is poorly developed and there are no paired appendages. 
—Lampreys and hagfishes. 
Series II. GNATHOSTOMATA. 
This includes all other vertebrates. They have usually two pairs 
of appendages, true jaws and a well developed skeleton. 
GRADE J. ICHTHYOPSIDA. 
Fish-like, breathe, at least while young, by gills, have paired ap- 
pendages, in the shape of legs or fins. In development there are never 
formed those structures to be described later as amnion and allantois. 
Class I. Pisces. 
Fishes respire permanently by gills developed in gill slits in the 
sides of the pharynx, have median and paired fins unless the latter be 
lost by degeneration. 
