xxii THE ANIMAL KINGDOM. 



In the crustaceans and insects we have an approach to true jaws, but here they 

 work laterally, not up and down, or vertically, as in the vertebrate jaws ; the mandi- 

 bles of these animals are modified feet, and the teeth on their edges are simply 

 irregularities or sharp processes, adapting the mandibles for tearing and comminuting 

 the food. It is generally stated that the numerous teeth lining the crop of Crustacea 

 and insects, serve to further comminute the food after being partially crushed by the 

 mandibles, but it is now supposed that these numerous points also act collectively as a 

 strainer to keep the larger particles of food from passing into the chyle-stomach until 

 finely crushed. 



The king-crab burrows in the mud for worms (Nereids, etc.) ; these may be found 

 almost entire in the intestine, having oply been torn here and there, and partly crushed 

 by the spines of the base of the foot-jaws, which thus serve the purpose effected by 

 the serrated edges of the mandibles of the genuine Crustacea and insects. 



Among vertebrates the lancelet is no better off than the majority of the coelen- 

 terates and worms, having no solid parts for mastication ; and the jaws and teeth of 

 the hag-fish, and even the lamprey eel, form a very different apparatus fi"om the jaws 

 and its skeleton in the higher vertebrates ; and even in the latter the bony elements 

 differ essentially in form in the different classes, though originating in the same man- 

 ner in embryonic life. In the birds, the jaw-bones are encased in horny plates; true 

 teeth being absent in the living species, the gizzard being, however, provided with 

 two hard grinding surfaces ; on the other hand, mammals without teeth are excep- 

 tional. 



The teeth of fishes are developed, not only in the jaws, but on the different bones 

 projecting from the sides and roof of the mouth, and extend into the throat. In 

 milny cases, in the bony fishes, these sharp recurved teeth serve to prevent the prey, 

 such as smaller fish, from slipping out of the mouth. On the other hand, the upper 

 and lower sides of the mouth of certain rays (Myliobatis) are like the solid pavement 

 of a street, and act as an upper and nether mill-stone to crush solid shells. 



In the toothless ant-eaters the food consists of insects, which are swallowed with- 

 out being crushed in the mouth; true teeth are wanting in the duck-bill, their place 

 being taken by the horny processes of the jaws, while in Steller's manatee the 

 toothless jaws were provided with horny solid plates for crushing the leaves of succu- 

 lent aquatic plants. Examples of the most highly differentiated teeth in vertebrates are 

 seen in those animals which, like the bear, are omnivorous, feeding on flesh, insects, and 

 berries, and which have the crown of the molars tubereulate ; while the canines are 

 adapted for holding the prey firmly as well as for tearing the flesh, and the incisors 

 for both cutting and tearing the food. 



Circulation. 

 Intimately associated with the digestive canal are the vessels in which the pro- 

 ducts of digestion mix with the blood and supply nourishment for the tissues, or, in 

 other words, for the growth of the body. In the Infusoria the evident use of the con- 

 tractile vesicles is to aid in the diffusion of the partly digested food of these micro- 

 scopic forms. In the Hydra the food stuff is directly taken up by the cells lining* the 

 coelom, while the imperfectly formed blood also finds access to the hollows of the 

 tentacles. The mode in which the cells lining the canals in the sponge take up, by 

 means of pseudopodia, microscopic particles of food, directly absorbing them in their 

 substance, is an interesting example of the mode of nourishment of the cellular tissues 



