xm 



PHYLUM CHORDATA 



but the rest of the canal is lined by a single layer of cells 

 underlaid by a layer of connective-tissue, the deeper part of which 

 is called the sub-mucosa ; epithelium and connective-tissue together 

 constitute the mucous membrane. The mucous membrane of 

 the stomach and sometimes of the intestine usually contains 

 close-set tubular glands ; those of the stomach, the gastric 

 glands, secrete gastric juice, which acts upon the proteid portions 

 of the food only ; the secretion of the intestinal glands digests 

 proteids, starch, and fats. Outside the mucous membrane are 

 layers of unstriped muscle, usually an internal circular and an 

 external longitudinal layer. Externally the intra-coalomic portion 

 of the canal is invested by peritoneum formed of a layer of connective- 



B 

 A 



gJT-% 



FIG. 778. A, longitudinal section of a tooth, semi-diagrammatic. PR, pulp-cavity ; PH', 

 opening of same ; ZB, dentine ; ZC, cement ; ZS, enamel. B, longitudinal section of 

 developing tooth. Eg, suhmucosa ; DS, dentine ; Ma, invaginated layer of enamel-organ ; 

 ME, epithelium of mouth ; O, odoutoblasts ; SK, stalk of enamel-organ ; ZK, tooth-papilla. 

 (From Wiedersheim's Vertebrata.) 



tissue next the gut and a single-layered ccelomic epithelium facing 

 the body-cavity. 



In connection with the enteric canal certain very characteristic 

 structures are developed. In the mucous membrane of the mouth 

 calcifications in most cases appear and form the teeth, which usually 

 occur in a row along the ridge of each jaw, but may be developed 

 on the roof of the mouth, on the tongue, and even in the pharynx. 

 A tooth is usually formed of three tissues dentine, enamel, and 

 cement. The main bulk of the tooth is made up of dentine (Fig. 778, 

 A, ZB), which occurs under three forms,. Hard dentine consists of 

 a matrix of animal matter strongly impregnated with lime-salts 

 and permeated by delicate, more or less parallel, tubules containing 



