xiii PHYLUM CHOllDATA 85 



from the mesenteron, and is therefore lined by an epithelium of 

 endodermal origin. The pharynx communicates with the exterior, 

 in Fishes and in the embryos "of the higher forms, by the gill-slits 

 {i. hr. a, 1-7); it communicates with the stomach by the 

 gullet. The stomach (st.) is usually bent upon itself in the 

 form of a TJ; the intestine (int.) is generally more or less 

 convoluted; hence the stomach and intestine are together con- 

 siderably longer than the enclosing abdominal cavity. In the 

 embryo the intestine is sometimes continued backwards into the 

 hffimal canal by an extension called the post-anal gut (p. a. g.), 

 which may perhaps indicate that the anus has shifted forwards 

 in the course of evolution. 



The epithelium of the buccal cavity is usually many-layered, 

 like that of the skin, of which it is developmentally an in -turned 

 portion; the pharynx and gullet have also a laminated epithelium, 

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

 laid by a layer of connective-tissue, the suh-mucosa ; epithelium 

 and sub-mucosa 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-coelomic 

 portion of the canal is invested by peritoneum formed of a 

 layer of connective-tissue next the gut and a single-layered 

 coelomic epithelium facing the body-cavity. 



In connection with the enteric canal certain very characteristic 

 structures are developed. In the mucous membrane of the mouth 

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

 occur m 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. 769, 

 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 con- 

 taining organic fibrils. Vaso-dentine is permeated with blood- 

 vessels, and consequently appears red and moist in the fresh 

 condition. Osteo-dentine approaches bone in its structure and 

 mode of development. The free surface of the tooth is usually 

 capped by a layer of enamel (ZS), a dense substance, either 

 structureless or presenting a delicate fibrillation, containing not 

 more than 3 to 5 per cent, of animal matter, and being, therefore, 

 the hardest tissue in the body. The cement (ZG) coats that 

 portion of the tooth which is embedded in the tissues of the jaw, 



VOL. II ^ 



