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Describe the Alimentary Canal and associated Organs. 

 State how developed. 



The enteron (alimentary canal or gut) of the higher 

 Craniata is developed mainly from secondary endoderm. 

 The greater portion, the mesenteron or mid-gut, is lined with 

 endoderm ; the front and terminal portions, stomodaeum 

 and proctodaeum, are lined with ingrowths of ectoderm. 

 Around this lining (the mucous membrane) the enteric 

 wall is formed from splanchnic mesoderm, and it consists 

 of muscles, connective tissue, blood-vessels, nerves, and an 

 outer covering of coelomic (peritoneal) epithelium. 

 - The gut is supported by folds of peritoneum (mesentery) ; 

 and its principal differentiated parts are the mouth or buccal 

 cavity (stomodaeum), pharynx (the first portion of the 

 mesenteron), gullet or oesophagus, stomach, intestine, anus. 

 In certain Fishes, in Amphibians, Reptiles, Birds, and in 

 Prototherian Mammals, the intestine ends in a cloaca, 

 into which the urinary and genital ducts also open. 



Associated with the buccal cavity are the teeth, the 

 tongue (an outgrowth from the buccal floor), and the salivary 

 glands, which secrete saliva for converting starch into 

 sugar. In higher Craniates the nasal sac opens into the 

 mouth, and usually (e.g., Birds and Mammals) there is a 

 palate between the upper nasal and the lower buccal cavity. 

 The hypophysis arises as an outgrowth from the buccal 

 roof and is subsequently constricted off. 



The walls of the pharynx have gill-clefts (8 or fewer). 

 The first pair of clefts are the spiracles of Elasmobranch 

 fishes, and become the Eustachian tubes in higher forms ; 

 the other clefts bear gills in Fishes and Amphibians, but 

 are vestigial in Reptiles, Birds, and Mammals. Outgrowths 

 from the pharynx are the thyroid gland (subsequently 

 constricted off), the lungs and the air-bladder (swim- 

 bladder or float), which in some fishes is shut off from 

 the pharynx. Buds from the epithelium of the visceral 

 clefts combine to form the thymus " gland." 



On the wall of the proximal or cardiac region of the 

 stomach there are gastric glands, which secrete the gastric 

 juice (pepsin and hydrochloric acid) for changing proteids 

 into soluble peptones. The food is moved onwards by 

 peristalsis (i.e., wave-like contractions of the muscles of 

 the wall). Until digestion is completed, the stomach is 

 kept closed at the distal end or pylorus by a sphincter 

 muscle. 



The bulky liver is a ventral outgrowth from the gut, 

 behind the stomach; its functions are: (1) the storing of 



