CHAPTER IX 



THE DENTITION, ALIMENTARY CANAL, AND DIGESTIVE GLANDS 



THE alimentary canal is a muscular tube with an epithelial 

 lining, formed for the reception and the digestion of the food. 

 It begins with a mouth, and from thence it extends backwards 

 through the coelom, finally communicating with the exterior 

 either by a cloacal or by an anal orifice. The oral or buccal 

 cavity into which the mouth leads is a stomodaeum, and is lined 

 by inpushed epidermis, while the hinder portion of the cloaca 

 and the anus are lined by a somewhat similar inpushing of the 

 epidermis which forms the proctodaeum. The rest of the 

 alimentary canal, consisting in succession of a pharynx, an 

 oesophagus, a stomach, and an intestine, constitutes the mesen- 

 teron, and is lined by endoderm. Teeth are developed from the 

 walls of the stomodaeum, and glands for the secretion of digestive 

 fluids from the endoderm of the meseiiteron. 



Dentition. 



In the Lampreys among the Cyclostomata teeth are developed 

 in the form of yellow conical structures on the inner surface of 

 the buccal funnel, and on the extremity of the rasping " tongue " 

 (Fig. 91, A). Each tooth consists of an axial papilla of the 

 dermis, sometimes enclosing a pulp -cavity, and invested by 

 the epidermis, and also by a stratified horny cone which forms 

 the projecting hard part of the tooth. The dermal papilla with 

 its ectodermal investment bears a superficial resemblance to the 

 germ of a true calcified tooth, but no odontoblasts are formed, 

 nor any calcic deposit, the laminated horny teeth being formed 

 by the gradual conversion of the successive strata of the 



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