X DIGESTIVE ORGANS 445 



The teeth, in Vertebrates generally, are developed in the 

 following manner. The ectodermal epithelium, of the 

 mouth (stomodeeum, p. 204) or in the case of the dermal 

 teeth of the dogfish that covering the body generally 

 grows inwards to form a ridge or dental lamina (Fig. 116, 

 S K) which projects into the underlying mesodermal con- 

 nective-tissue and becomes enlarged distally to form a bell- 

 shaped enamel -organ, into the base of which a mesodermal 

 dental papilla (Z K) extends: the superficial part of this 

 papilla forms a layer of cells known as odontoblasts (O). 

 The dentine (DS) is formed, in successive layers, from the 

 odontoblasts, and gradually accumulates between them and 

 the epithelium lining the interior of the enamel organ (Ma), 

 which gives rise, also in successive layers, to the enamel, 

 the hardest, external part of the tooth. Around the base 

 of the papilla more or less bone-like substance the cement 

 is formed (compare p. 510). The apex of the tooth 

 eventually breaks through the overlying epithelium and 

 comes to the surface. It will thus be seen that while the 

 teeth are mainly mesodermal structures, a part of them 

 the enamel--is ectodermal in origin. 



The mouth leads into an oral cavity on the floor of 

 which is a rudimentary tongue (Fig. 117, tng) capable of 

 very little movement which passes insensibly into the 

 throat or pharynx (ph), distinguished by having its 

 walls perforated by five pairs of slits, the internal branchial 

 apertures (i. br. a), as well as by the inner openings of 

 the spiracles (sp). The pharynx is continued by a short 

 gullet (gul) into a capacious U-shaped stomach, con- 

 sisting of a wide cardiac division (cd. st) and a narrow 

 pyloric division (Pyl. st). The pyloric division communi- 

 cates by a narrow valvular aperture with the intestine 

 (int), a wide, nearly straight tube having its lining 

 membrane produced into a spiral fold, the spiral valve 

 (sp. vl), which practically converts the intestine into a 

 very long, closely-coiled tube, and greatly increases the 

 absorbent surface. Finally the intestine opens into a 

 large chamber, the cloaca (cl), which communicates with 

 the exterior by the vent. 



From the gullet backwards the enteric canal is con- 



PRACT. ZOOL. f f 



