DEVELOPMENT OF THE DIGESTIVE TUBE 



617 



RECTAL 

 PYLORIC INTESTINE RECTUM GLAND 



VALVE 



GILL OPENINGS 



DUODENUM 



Fig. 291. Diagrams of intestinal tracts in various fishes. (Redrawn from Dean, 1895, 

 Fishes, Living and Fossil, Macmillan, N. Y.) (A) Petromyzon, the cyclostome. (B) 

 Protopterus, the lungfish. (C) The shark. 



of epidermal cells occurs along the edge of the forming mouth. The labial 

 groove then divides the edge of the forming mouth into an outermost lip margin 

 and the gum or jaw region (fig. 288A). In forms where the lip is mobile, the 

 lip region becomes highly developed and the muscle tissue which invades this 

 area comes to form the general mass of the lip. 



7) Oral Glands. Mouth glands are present throughout the vertebrate series. 

 Mucus-secreting glands are the predominant type, but specialized glands, pro- 

 ducing special secretions, appear in many instances. The cyclostomatous fish, 

 for example, possesses a specialized gland which secretes an anticoagulating 

 substance to prevent coagulation and stoppage of blood flow in the host fish 

 to which it may be temporarily attached by its sucker-like mouth. Mean- 

 while, it rasps the host's flesh with its horny teeth and "sucks" the flowing 

 blood. Salivary glands (i.e., glands forming the saliva) make their appearance 

 in the amphibia. Such glands may be found on the amphibian tongue, where, 

 as lingual glands, they secrete mucus and a watery fluid. Intermaxillary glands 

 are present on the amphibian palate. The poison glands of the Gila monster 

 and of snakes represent specialized oral glands (fig. 290A). Salivary glands 

 are present also below the tongue and around the lips and palate in snakes. 

 Birds, in general, possess salivary glands of various sorts. The mammals are 

 characterized by the presence of highly developed, salivary glands, among 

 which are the parotid, sublingual, and submaxillary glands. Unlike most of the 

 salivary glands in other vertebrates, the mammalian salivary glands, in many 

 species, secrete mucus and a watery fluid, together with a starch-splitting 

 enzyme, ptyalin. 



The submaxillary and sublingual glands in mammals arise as evaginations 

 of the oral epithelium in the groove between the forming lower jaw and the 



