78 ZOOLOGY 



SECT. 



fatty tissue called marrow. The extremities become simply 

 calcified in the lower forms, but in the higher a distinct centre of 

 ossification may appear in each, forming the epipliysis, which finally 

 becomes ankylosed to the shaft. 



Digestive Organs. The enteric canal is divisible into buccal 

 cavity (Fig. 715, A, buc. c.), pharynx (ph.), gullet, stomach (st.), and 

 intestine (int.), the latter sometimes communicating with the 

 exterior by a cloaca (cl.), which receives the urinary and genital 

 ducts. The buccal cavity is developed from the stomodseum of 

 the embryo : the proctodseum gives rise to a very small area in 

 the neighbourhood of the anus, or, when a cloaca is present, to its 

 external portion : all the rest of the canal is formed from the 

 mesenteron, and is therefore lined by an epithelium of endo- 

 dermal origin. The pharynx communicates with the exterior, in 

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

 (i. br. a. 1-7) ; it communicates with the stomach by a compara- 

 tively narrow gullet. The stomach (st.) is usually bent upon itself 

 in the form of a U; 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 

 haemal canal by an extension called the post-anal gut (p. a. g.\ 

 which may be taken to 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 (Fig. 723, E) 

 underlaid by a loose layer of connective tissue, the sub-mucosa (Z) ; 

 epithelium and sub-mucosa together constitute the mucous mem- 

 brane. The mucous membrane of the stomach and sometimes of 

 the intestine contains close-set tubular glands (D) ; those of the 

 stomach, the gastric glands, secrete gastric juice, which acts upon 

 the proteid portions of the food only ; the intestinal glands digest 

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

 layers of unstriped muscle, usually an internal circular (M') and 

 an external longitudinal (M ) 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 

 calcifications 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 



