HOW ANIMALS EAT 



267 



The number of these plates varies greatly ; the garden 

 slug has 1 60 rows, with 180 teeth in each row. 



All living birds, and some other vertebrates, as ant- 

 eaters, 78 turtles, tortoises, toads, and sturgeons, are with- 

 out teeth. Their place is often supplied by a horny 

 beak, a muscular gizzard, or both structures. 



In a few vertebrates, horny plates take the place of 

 teeth, as the duck mole (Ornithorhynchus) and whalebone 

 whale. In the former, the plates consist of closely set 

 vertical hollow tubes ; in the latter, the baleen, or whale- 

 bone, plates, triangular in shape, and fringed on the 

 inner side, hang in rows from the gums of the upper 

 jaw. In some whales there are about 300 plates on each 

 side. 79 



True teeth, consisting mainly of a hard, calcareous 

 substance called dentine, are found only in backboned 

 animals. They are distinct from 

 the skeleton, and differ from 

 bone in containing more mineral 

 matter, and in not showing, under 

 the microscope, any minute cav- 

 ities, called lacuna. A typical 

 tooth, as found in man, consists 

 of a central mass of dentine, 

 capped with enamel, and sur- 

 rounded on the fang with cement. 

 The first tissue is always pres- 

 ent, while the others may be 



T T , , f 



absent. It is a mixture of am- 



mal and mineral matter disposed 



in the form of extremely fine 



tubes and cells, so minute as to prevent the admission 



of the red particles of blood. One modification of it is 



ivory, seen in 'the tusks of elephants. Enamel is the 



hardest tissue of the body, and contains not more than 



FIG. 229 . Section of Human MO- 



lar, enlarged: k, crown; n, 



nec ' k . /( . ,; enamel . ^ 

 dentine; c > cement: > P UI P 



cavity. 



