60 ZOOLOGY SECT, xni 



CLASS IV. REPTILIA, 

 Including Lizards, Snakes, Crocodiles, Turtles, and Tortoises. 



CLASS V. AVES, 

 Including Birds. 



CLASS VI. MAMMALIA, 



Including Hairy Quadrupeds, Seals, Whales, Bats, Monkeys, and 



Man. 



External Characters. The body of Craniata (Fig. 712) is 

 bilaterally symmetrical, elongated in an antero-posterior direction, 

 and usually more or less cylindrical. It is divisible into three 

 regions : the head, which contains the brain, the chief sensory 

 organs, and the mouth and pharynx ; the trunk, to which the 

 coelome is confined, and which contains the principal digestive and 

 circulatory as well as the excretory and reproductive organs ; and 

 the tail, or region situated posteriorly to the coelome and anus, and 

 containing no essential organs. Between the head and trunk 

 there is frequently a narrow region or neck, into which the coelome 

 does not extend. In aquatic Vertebrates the tail is of great size, 

 not marked off externally from the trunk, and is the chief organ 

 of locomotion : in terrestrial forms it becomes greatly reduced in 

 diameter, and has the appearance of a mere unpaired posterior 

 appendage. 



The mouth (mth.) is a transverse aperture placed at or near the 

 anterior end of the head. Near it, sometimes dorsal, sometimes 

 ventral in position, are the paired nostrils or anterior nares (na.) 

 or in Cyclostomata the single nostril leading to the organs of 

 smell. Farther back, on the sides of the head, are the large paired 

 eyes (e.}, and on the dorsal surface there is sometimes more or less 

 indication of a vestigial median or pineal sense organ (pn. e.), which 

 may take the form of an eye. Posterior to the paired eyes are 

 the auditory organs (au.), the position of which is indicated in the 

 higher forms by an auditory aperture. 



On the sides of the head, behind the mouth, are a series of 

 openings, the gill-slits or external branchial apertures (e. br. a. 1 

 7) : they are never more than seven in number, and in air- 

 breathing forms disappear more or less completely in the adult. 

 In the higher Fishes a fold called the operculum (Fig. 726, op.) 

 springs from the side of the head immediately in front of the 

 first gill-slit and extends backwards, covering the branchial 

 apertures. 



On the ventral surface at the junction of the trunk and tail is 

 the anus (an.). Distinct urinary and genital apertures, or a single 



