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CHAPTER XXVI 



THE ALIMENTARY SYSTEM 



The alimentary system comprises the tube which leads from 

 mouth to anus, together with the glands attached to it which 

 aid in the processes of digestion. There is a slight invagination 

 of the ectoderm at the mouth and anus, forming the stomodaeum 

 and the proctodeum ; but the remainder, which forms by far 

 the larger part of the alimentary system, is formed exclusively 

 from the endoderm. In addition to the digestive glands, the 

 alimentary canal has a number of derivatives which have been 

 considered in connexion with other organ-systems. So the 

 gill-pouches and the larynx and lungs belong to the respiratory 

 system ; the allantoic bladder forms part of the excretory 

 system ; while the thyroid gland, which in Gnathostomes and 

 adult Cyclostomes is one of the endocrine organs, belongs to 

 the alimentary system in Amphioxus and the larval Cyclostome 

 (Ammocoete). 



The primitive method of obtaining food is by the creation 

 of a current of water towards the mouth by means of cilia. 

 This is the case in Amphioxus (and the Ascidians). Here the 

 endostyle is accessory to the alimentary system in that it 

 ensures that the particles of food reach the intestine instead of 

 being lost with the current of water flowing out through the 

 gill-slits. The method of feeding by means of a sucking 

 mouth and a rasping tongue which is characteristic of the 

 Cyclostomes, is secondary and specialised. In all the Gnatho- 

 stomes, the most anterior visceral arches, between the mouth 

 and the first visceral cleft, become modified and adapted for 

 seizing food, and give rise to the jaws. This method enables 

 food of larger size to be obtained than is possible by the ciliary 



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