ALIMENTARY CANAL. 125 



corpuscles of the blood; sometimes the form of simple tissues, as in the 

 epidermis, which protects the free surfaces of the body. Very often it ap- 

 pears as distinct organs, like the heart or the lungs. But as a rule the 

 mechanism consists of a number of different tissues or organs (or both), 

 which work together for a common end, thus constituting a system. 



SPECIAL MORPHOLOGY. 



Alimentary System. This consists of the inner body-tube 

 or alimentary canal, which is divisible into several differently 

 constructed portions playing different parts in the process of 

 alimentation. Going backwards from the mouth these are as 

 follows : 



1. The pharynx (Fig. 75, j9/i), an elongated barrel-shaped 

 pouch extending to about the 6th somite. Its walls are thick 

 and muscular, and from their coelomic surface numerous small 

 muscles radiate on every side to the body-wall. When these 

 muscles contract, the cavity of the pharynx is expanded ; and if 

 the mouth has been previously applied to any solid object, such 

 as a leaf or pebble, the pharynx acts upon it like a suction-pump. 

 In this way the animal lays hold of the various objects, nutritious 

 and otherwise, which it devours or draws into its burrow. 



2. The (Ksopliagus (a?), a slender, thin-walled tube extending 

 from the 6th to the 15th somite. Through this the food is 

 swallowed, being driven slowly along by wave-like (peristaltic) 

 contractions (p. 129). In the region of the llth and 12th somites 

 are three pairs of small pouches opening at the sides of the oeso- 

 phagus. These are the calciferous glands (c.yl). They contain 

 solid masses of calcium carbonate, and Darwin conjectures that 

 their use is partly to aid digestion by neutralizing the acids gen- 

 erated during the digestion of leaves, and perhaps partly to serve 

 as an outlet for the excess of lime in the body, especially when 

 worms live in calcareous soil. 



3. The crop (e), about the 16th somite ; a thin-walled, sac- 

 like dilatation of the alimentary canal, which serves as a reservoir 

 to receive the swallowed food. 



4. The gizzard (g\ about the 17th somite ; a cylindrical, 

 firm and muscular portion, lined by a horny membrane. In this 

 the food is rolled about, squeezed and ground to prepare it for 

 digestion. 



