TEETH. 41 



(1) Labrum pronounced; mandibles two hard, generally 

 robust organs each composed of a single piece and commonly 

 denticulate upon opposing surfaces. Maxillae two softer op- 

 posing organs, furnished with palpi, each composed of one to 

 six articles. Labium, a single inferior piece, furnished with 

 labial palpi, each composed of from one to four articles. 

 Ligula placed at the base of the labium. Includes Cole- 

 optera (beetle), Neuroptera (dragon-fly), Orthoptera (grass- 

 hopper). (2) The suctorial form of mouth may be due to 

 the labium being prolonged into a grooved proboscis, with 

 the other parts of the mouth appearing as setae toward its 

 base, as in Diptera (fly); or, to the labium preserving the 

 function of a sheath to the setiform mandibles and max- 

 illae, the labrum becoming rudimentary, as in Hemiptera 

 (bug) ; or, the labium may be absent, with the mandibles ru- 

 dimental, while the maxillae are enormously developed into 

 a symmetrical suctorial canal, furnished at base with small 

 maxillary palpi, the whole coiled and protected in rest by 

 palpi arising from labium, as in Lepidoptera (butterfly). 



Apparently intermediate between these two types appear 

 the organs of Hymenoptera (bee), in which the maxillae are 

 mostly elongate, and form with labium a kind of sucking 

 proboscis; at the same time the mandibles are well developed, 

 but are used more for cutting and gnawing during search for 

 food than in its preparation. 



Crustacea. Jaws when present are modified feet. In 

 Limulus (king-crab), the feet perform function of jaws. In 

 Decapoda (lobsters, crabs, etc.), an upper lip, an upper pair 

 of jaws, furnished with palpi, two pairs of lower jaws, each 

 with a palpus, are present. 



VERTEBRATA. The tooth of a vertebrate animal consists 

 of a cellular and tubular basis of animal matter, containing 

 earthy particles, a fluid, and a vascular pulp. (Owen.) 



The basis is dentinal in typical tooth; corneous in certain 

 modified structures, as in teeth of marsipobranchiate fishes, 

 in Balcena (whale-bone whale), and Ornithorhynchus (duck-bill). 



A typical tooth possesses a central dentinal mass, capped 

 with enamel, and surrounded by cementum. Cementum 



^ 



TJHIVBRSIT7 



