VERTEBRATA. 73 



the mucous membrane folded longitudinally so as to form a surface 

 of considerable extent. The rectum dilates into a cloaca which 

 receives the openings of the urinary and genital organs. 



AVES. 



Nothing can be more beautiful to observe than the rigid economy 

 which is displayed in the accurate adaptation of the digestive 

 apparatus of birds to the various and dissimilar kinds of iood on 

 which, from their diversified living habits they are destined to subsist. 

 The absence of teeth in this class is supplied by strong horny 

 beaks and powerful muscular gizzards, the former performing the 

 part of cutting, and the latter of grinding teeth ; the form of the 

 bill will vary according to the food of the different species of birds 

 and their mode of procuring it, thus in the climbing frugivorous 

 maccaws, parrots, and cockatoos, it is broad and powerful to break 

 the hard shelly coverings of seeds, and most of the granivorous 

 order have a similar structure. The broad bills of ducks, geese, 

 and other aquatic species are well adapted for obtaining worms and 

 other substances from watery or muddy situations, whilst the eagle, 

 the hawk, the owl, and other rapacious birds have short, strong, 

 arched, dense bills, with cutting edges, equally suited to seize, cut, 

 or tear their living prey. The tongue, which in birds serves the 

 purpose of a prehensile organ, is as variable in form as the bills, 

 being long, broad, and covered with recurved spines in the swans; 

 short, round, and flexible in the parrots and cockatoos, and short 

 and muscular in the struthious birds. The tongue of the flamingo 

 is very remarkable, it is composed of an elastic, cellulo-fatty sub- 

 stance, its form is nearly cylindrical, the pointed apex being sup- 

 ported by an osseous plate iuferiorly. A deep groove runs along 

 the centre of the upper surface with a row of recurved spines on 

 either side. The os hyoides in this class very much resembles that 

 of reptiles, and the length of its glosso-hyal element chiefly deter- 

 mines the length of the tongue. 



Since the food for birds remains but a short time in the mouth 

 and undergoes very little change there, their salivary glands are 

 small. In the crow they consist of a series of conical follicles 

 situated along the sides of the mouth, and opening separately on 

 its mucous surface; in most other birds, however, there are four 

 pairs, one under the tongue, another at the junction of the angles 

 of the lower jaw, another close to the cornua of the os hyoides, and 

 the fourth is placed at the angles of the mouth: they are most 

 developed in the frugivorous species. The uvula and velum are 

 not present, and the narrow laryngeal aperture is protected by the 

 retroverted papillae at the base of the tongue, except in the coot, the 

 albatross, and a few others where the epiglottis exists merely in a 

 rudimentary form quite insufficient to cover the opening. 



There is a remarkable pouch under the jaw of the pelican which 

 serves as a net for seizing fish, and is capable of containing ten 



