XIII 



PHYLUM CHORDATA 65 



CLASS VI. MAMMALIA, 



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



Man. 



External Characters. The body of Craniata (Fig. 766) 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 

 ccelome 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 ccelome and anus, 

 and containing none of the more essential organs. Between the 

 head and trunk there is frequently a narrow region or neck, into 

 which the ccelome 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 usually 

 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 Cyclostomi 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 (an.), 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 ?) : they are rarely more than seven in number, (Cf. p. 139) and in 

 air-breathing forms disappear more or less completely in the adult. 

 In the higher Fishes a fold called the operculum (Fig. 780, 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 

 urino-genital aperture, are sometimes found either in front of or 

 behind the anus, but more commonly the urinary and genital ducts 

 open into the termination of the enteric canal, or cloaca, so that 

 there is only a single egestive opening, known as the cloac'il 

 aperture. On either side of this there may be a small abdominal 

 pore (ab. p.) leading into the ccelome. 



In Fishes and some Amphibians, the trunk and tail are produced 

 in the middle dorsal line into a vertical fold or median fin, which is 



