ANNELIDA OR ANNULATA 107 



from the 8th somite to the last one, which permit the passage of 

 fluids from the coelom to the skin. The anus is at the posterior end. 



Skin. — The thin protecting cuticle is formed from the living cells 

 of the hypodermis beneath. The hypodermis consists of a single 

 layer of cells most of which are covering and supporting, but some 

 of which are modified into glandular (mucus) cells, and others into 

 nervous cells. The nerve cells are connected with sensory fibers 

 passing into the nerve cord, and the animal is very sensitive to 

 light, touch and chemically different substances. The setae which 

 are chitinous are worn away and replaced by reserve setae that 

 grow from the main seta-sac. 



Muscular System. — The circular muscles lie immediately beneath 

 the hypodermis and, contracting, elongate the segments. The 

 longitudinal muscles, contracting, draw the ends of the segments 

 towards each other, and the direction of the setae determines whether 

 the movement is forwards or backwards. 



Body Cavity. — The body cavity, lined by peritoneum, contains 

 the digestive tract, gonads, nephridia, circulatory system and nerv- 

 ous system beside the coelomic fluid with yellow cells, derived 

 from the walls of the intestine, and the phagocytic amebocytes which, 

 like the phagocytic white corpuscles of man, engulf poisonous 

 particles. 



Digestive System. — The mouth or buccal cavity (1-3 somites) 

 leads into the pharynx (4-5 somites), with glands which moisten, 

 and with powerful muscles which force food on. External muscles 

 attached to the body wall expand the pharynx. The esophagus 

 (6-i4th somite) has three pairs of saccular calciferous glands, at the 

 loth, nth, and 12th somites, secreting calcium carbonate, which 

 neutralizes the free acid of the soil. The secretions from the 

 posterior pairs of calciferous glands open into the anterior pair, and 

 thence into the esophagus. (Figure 46.) 



The three pairs of glands are really parts of one glandular struc- 

 ture, which extends from somites 10 to 14. In many specimens of 

 Lumbricus terrestris, the only distinct enlargements are in somite 10. 

 The crop (15th and i6th somites) is for storage, and mixture with 

 the secretions of the calciferous glands. The gizzard (17th and i8th 

 somites) grinds the food with sand and gravel. The stomach- 

 intestine (19th somite to anus) has a median dorsal infolding, the 

 typhlosole, that increases the surface for absorption and retards the 

 passage of food. 



