XII 



PHYLUM CHORDATA 



343 



ZS 



with the myomeres), indicates that the body, like that of 

 Nereis or an Arthropod, is metamerically segmented. In 

 the lizard and rabbit the metamerism of the muscular sys- 

 tem, though distinguishable at an early stage, becomes lost 

 in the adult, and the muscles take on a much more compli- 

 cated arrangement. 



On the jaws are a series of teeth, the function of which 

 is to seize the food, and in the rabbit cut it into fragments, 

 and crush it into yet smaller particles, in order to prepare it 

 for the process of digestion. In the dogfish the teeth are 

 numerous and of uniform character throughout, small with 

 sharp points directed backwards. At 

 their bases they are fixed to the surface 

 of the cartilage of the jaw by means of 

 dense fibrous tissue. In the lizard the 

 teeth are also of uniform character 

 (homodont dentition]. They are of a 

 simple conical shape, and fixed to the 

 bone of the jaws. In the rabbit the 

 teeth are distinctly visible into sets, dif- 

 fering from one another in shape and 

 function (heterodont dentition). Their 

 bases are lodged in sockets or alveoli in 

 the substance of the jaws. 



The structure of the tooth is the same 

 in all three cases. The main mass of 

 the tooth consists of dentine, a densely FIG. 215. Longitudinal 



, .- , . , . . . ,. section of a tooth, semi- 



Calcified material permeated by delicate diagrammatic. PH, 

 parallel tubules. The free surface is ' 



ze 



sheim>s Vertebrata ^ 



covered with a layer of still harder mate- 



rial, the enamel, and the basal portion is 



covered with a layer of cement, which is similar in micro- 



scopic structure to bone. 



