FISHES. 



227 



In the brain the separate divisions are subequal in size. 

 The vagus nerve innervates all but the anterior pair of gill slits, 

 and in the aquatic forms bears a large ramus lateralis distrib- 

 uted to the sense organs of the skin. The eleventh nerve is 

 apparently a branch of the vagus, while the twelfth is repre- 

 sented by the first, or first and second, spinal nerves. 



The heart, usually far forward in position, has the sinus 

 venosus external to the atrium, the atrium single or divided by a 

 longitudinal septum into right 

 and left auricles, the ventricle 

 always simple and undivided. 

 In the lower forms the conus 

 is large and well developed ; in 

 the teleosts it is reduced to a 

 row of valves between the ven- 

 tricle and the bulbus. At least 

 one pair of aortic arches per- 

 sists in a complete state in 

 the adult, while some or all are 

 permanently or temporarily 

 (amphibia) connected with 

 the gills. A renal portal sys- 

 tem (p. 194) occurs ; and the 

 red blood corpuscles are large, oval, and nucleated. The func- 

 tional kidney is the mesonephros, although rarely the proneph- 

 ros persists. 



The alimentary canal is comparatively short, and either 

 terminates in a cloaca (p. 39), or the vent is anterior to the 

 urogenital openings. No metanephros is developed, and the 

 ova are frequently large. 



The Ichthyopsida are divisible into Pisces and Amphibia. 



Class I. Pisces. 



Ichthyopsida with persistent gills ; paired appendages al- 

 most always present in the shape of fins ; median fins sup- 

 ported by dermal rays ; body usually covered with dermal 

 scales ; postcava. Eustachian tube, and stapes lacking. Nos- 

 trils (except in dipnoi) never opening into the mouth. 



A 



Fro. 233. Relations of the conus, 

 A, in elasmobranchs and ganoids; B, 

 in teleosts. a, auricle ; b, bulbus ar- 

 teriosus; c, conus, reduced in jS to a 

 circle of valves ; v, ventricle. 



