xii PHYLUM CHORDATA 341 



there will be found immediately beneath it a thick layer of 

 muscle. This is distinctly divided into segments or 

 myomeres similar to those of Amphioxus, and this, with the 

 division of the vertebral column into segments or vertebrae 

 (which however do not exactly correspond in arrangement 

 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 system, 

 though distinguishable at an early stage, becomes lost in the 

 adult, and the muscles take on a much more complicated 

 arrangement. 



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

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

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

 for the process of digestion. In the Dog-fish 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 divisible into sets, 

 differing 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 (Fig. 201) is similar in all three 

 cases. The main mass of the tooth consists of dentine (ZZ?.), 

 a densely calcified material permeated by delicate parallel 

 tubules. The free surface is covered with a layer of still 

 harder material, the enamel (ZS.\ and the basal portion is 

 covered with a layer of cement (ZC.), which is similar in 

 microscopic structure to bone. 



The anterior part of. the cavity into which the mouth 



