IX DIGESTIVE ORGANS 429 



placoid scales or dermal teeth enlarged for the purpose 

 of seizing prey, and are continually renewed on the inner 

 sides of the jaws as they are worn away on the outer sides. 



The teeth, in Vertebrates generally, are developed in the following 

 manner. The ectodermal epithelium of the mouth (stomodaiini, p. 204) 

 — or in the case of the dermal teelh of the dogfish that covering the body 

 generally — grows inwards to form a ridge or dental lamina (Fig. 107, 

 SK) which projects into the underlying mesodermal connective tissue and 

 becomes enlarged distally to form a bell-shaped enamel-organ, into the 

 base of which a mesodermal dental papilla [ZK] 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, or hardest part of the tooth. Around the base 

 of the papilla more or less bony matter — the cement — is formed. 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. 108, tug) capable of 

 very little movement, and which passes insensibly into the 

 throat or pharynx {ph), distinguished by having its walls 

 perforated by five pairs of slits, the internal branchial aper- 

 tures (/. br. a) as well as by the inner opening of the 

 spiracle (sp). The pharynx is continued by a short gullet 

 (gul) into a capacious, U-shaped stomach, consisting of a 

 wide cardiac division {cd. st) and a narrow pyloric division 

 {pyl. st). The pyloric division communicates 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. 



