4 INTRODUCTION 



The heart consists of two successive chambers, an auricle (atrium) 

 and a ventricle, and in forms which respire by means of gills, con- 

 tains 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 (coelom) which in the adult does 

 not extend into the head. Each viscus is supported by a fold (mesen- 

 tery) 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, 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. 



Class I. Marsipohranchii 

 Characters as above. 



Sub-class I. Petromyzo files {Hyperoarlia) 



Well-developed dorsal fin ; naro-hypophysial duct on top of head ; 

 seven gill openings on either side.' — ^Lampreys. 



Sub-class II. Myxinoidei (Hyperolretia) 



Dorsal fin small or absent; naro-hypophysial duct at tip of snout; 

 either a single gill opening on either side, or from six to fourteen 

 present. — Hagfishes. 



Series II. GNATHOSTOMATA 



This includes all other vertebrates. They have usually two pairs 

 of appendages, paired nostrils, true jaws and a well-developed skeleton. 



