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. 



SERIES 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 I. 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. 



