II. Organs. 5. Sense Organs. 6. Alimentary Ganal. 23 



for example, of Earthworms, which withdraw with the greatest 

 rapidity into their holes if a strong light is thrown upon their anterior 

 ends, where the cerebral ganglion is situated. 



6. Alimentary Canal. 



Like the Amoeba, Metazoan cells undergo constant metabolic 

 change with consequent loss of material, and this is a necessary 

 condition for the continuance of the vital processes. In order to 

 make good this loss, they must feed. Unlike the Amoeba, however, 

 the individual cells are unable to obtain food direct from the 

 environment : there must be some special arrangements for nutrition : 

 hence the development of an alimentary canal. Pood is taken 

 into this canal and there digested, i.e., reduced to a fluid state, so 

 that it can be absorbed by the wall of the digestive tract, and carried 

 to the various tissues of the body. Those portions which are not 

 digested and absorbed pass out again (excreta) . 



Whilst in most animals, secretions, by means of wMch the food is digested, 

 are poured into the alimentary canal, there is another ai-rangement amongst 

 Ooelentera and Sponges. No such digestive juices are found here, but the food 

 comes in contact with epithelial cells, and is digested and absorbed directly by 

 them. Smaller organisms, e.g., Diatoms, may even be enthely engulfed by the 

 epithehal cells. This also frequently occurs among Plat-worms. 



In its simplest form the digestive tract is a sac or a canal, which 

 communicates with the exterior by a single aperture only. This 

 opening then serves both as an entrance or mouth, and also as an exit 

 for the undigested portions of the food (Ooelentera, Flat- worms). 



In most animals, however, the digestive tract has two openings, a 

 mouth and an a n u s. It is then usually a long tube, with the 

 mouth at one end, and the anus at the other. It is frequently 

 divided into several regions, with different functions. In the simplest 

 cases, only three regions can be distinguished, the fore-gut 

 (stomodseum), which is often very muscular, and serves in different 

 ways for the ingestion of food: the mid- gut (mesenteron) , which 

 is usually long, and in which digestion and absorption go on; and 

 the hind-gut (proctodseum) , which serves as an excurrent canal, 

 and as a reservoir for undigested materials (Nematodes, Annelids). 

 In other cases, the fore-gut is again divided into a large buccal 

 cavity, often provided with organs for the prehension and retention 

 of prey, or for the comminution of food (teeth) ; and a narrow 

 oesophagus. The mid-gut is frequently, as in Vertebrata, 

 divided into a capacious anterior portion, the stomach, and a 

 long narrow hinder part, the true gut or intestine (small intestine 

 of Vertebrata). The stomach is digestive, its walls are beset with 

 glands, which secrete a digestive fluid; whilst the intestine is 

 absorptive, though digestion goes on here also. More rarely the 

 hind-gut is also divided into several sections. Occasionally some 



