232 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



In this connexion mention may be made of the baleen or 'whale- 

 bone' of the balenid whales. This takes the form of large plates of 

 horny material, attached in series to the margins of the upper jaw, so 

 that with their fringed ends and edges they serve as strainers to ex- 

 tract the plankton (minute floating life) from the sea. This baleen 

 is formed by the agglutination of enormously developed cornified 

 papillae. 



Egg Teeth. — In the embryos of certain lizards and snakes one of the median 

 teeth of the first dentition of the premaxillary region projects from the mouth 

 and is used for the rupture of the egg shell, thus allowing the escape of the 

 young. In the turtles, Sphenodon, crocodiles, birds, and monotremes an egg 

 tooth is formed on the upper surface of the beak which is used for the same 

 purpose. However, it differs greatly as it is but a thickening, often calcified, 

 of the epidermis (fig. 218). 



The Tongue 



The tongue as it occurs in its more primitive condition in the fishes 

 is merely a fleshy fold developed from the floor of the mouth between 

 the hyoid and mandibular arches, the hyoid frequently extending into 

 and supporting it. It is incapable of motion, except as moved by the 

 supporting skeleton, for it lacks intrinsic muscles. It is sensory, 

 having both tactile and gustatory functions. It is often papillose, 

 and in a few teleosts it bears teeth. 



The tongue in the cyclostomes is considerably different. Here it 

 is thick and fleshy and is supported by a cartilaginous skeleton (p. 

 84) and is moved by appropriate protractor and retractor muscles at 

 the base, developed from the postotic myotomes and innervated by 

 the hypoglossal nerve. With its terminal armament of epidermal 

 teeth it serves as the boring organ with which the myxinoids obtain 

 entrance into their prey, while in the lampreys it serves as a rasping 

 organ and also as part of the sucking apparatus. 



In the amphibia there is a greater range of structure. In a few 

 anura (aglossa) the tongue is practically absent; in the perenni- 

 branchs it is scarcely more advanced than in the fishes, but elsewhere 

 it contains intrinsic muscles (innervated by the hypoglossal nerve) and 

 is extremely mobile. It consists of a small basal portion corresponed 

 ing to the tongue of the fish, to which is added a large glandular 

 part arising between the copula and the lower jaw. This secretes 

 the slime, so useful in capturing the prey. In the anura the tongue 

 is attached at the margin of the jaw, its- free end, when at rest, 

 being folded back on the floor of the mouth. In urodeles the base 



