232 CLASSIFICATION OF VERTEBRATES. 



however, may become connected (many teleosts) with the otic 

 region by a chain of bones. The skeleton of the fins themselves 

 varies greatly, but there is never anything that approaches the 

 pentadactyle condition found in the higher vertebrates. 



The heart always consists of a single atrium and a single 

 ventricle, and a sinus which is undivided except in the dipnoi. 

 Conus and bulbus vary in their development. The aortic arches 

 are always symmetrically developed. No postcava occurs ; the 

 blood from the posterior portion of the body being returned by 

 the postcardinals, the portal system, and the hypogastric veins. 

 A spleen is always present. 



The functional excretory organ is the mesonephros, the pro- 

 nephros but rarely persisting. Urinary bladders are frequently 

 present ; but these are simply expansions of the urinary duct, 

 and are not comparable to the allantoic bladder of the higher 

 vertebrates. The urinary ducts either empty into the hinder 

 end of the intestine or by separate openings to the exterior. 

 Pori abdominales are almost always present, and these in some 

 cases serve for the extrusion of the reproductive products. Only 

 in elasmobranchs and ganoids do the urogenital ducts serve to 

 carry away the eggs and milt. 



SUB-CLASS I. ELASMOBRANCHII (PLAGIOSTOMI, 

 CHONDROPTERYGII). . 



Cartilaginous fishes without true bones ; tail usually hetero- 

 cercal ; gill slits 5-7, no operculum present ; skin, with rare ex- 

 ceptions, bearing placoid scales ; pelvic fin in males of recent 

 forms bearing a complicated copulatory apparatus ( ' clasper ' ) ; 

 skull, amphi- or hyostylic, never autostylic ; upper jaw formed 

 by pterygoquadrate ; a true optic chiasma ; intestine with a spiral 

 valve ; no air-bladder. 



The sub-class elasmobranchii includes those fishes popularly 

 known as sharks and skates, all of which, -with two or three 

 exceptions, are marine. The body is usually fish-like in shape, 

 but is greatly flattened horizontally in the skates. The caudal 

 fin, when present, is always heterocercal ; but in many skates the 

 fin is absent, the tail tapering to a point. In most forms the 

 mouth and nostrils are ventral in position, and are placed some 



