THE EARTHWORM AND ANNELIDS IN GENERAL 169 



neum, a thin stratum of flattened cells lining the coelom. The 

 gland cells of the epidermis (Fig. 86) are called " goblet cells " 

 because of their shape; they secrete the mucus which helps to 

 keep the surface of the worm moist. 



Digestive System. The alimentary canal (Fig. 87) consists 

 of (i) a mouth cavity or buccal pouch in somites I to III, (2) a 

 thick muscular pharynx (ph.) lying in somites IV and V, (3) a 

 narrow straight tube, the oesophagus (ozs.) which extends through 

 somites VI to XIV, (4) a thin-walled enlargement, the crop or 

 proventriculus (cr.), in somites XV and XVI, (5) a thick mus- 

 cular-walled gizzard (giz.) in somites XVII and XVIII, and (6) a 

 thin- walled intestine (int.) extending from somite XIX to the anal 

 aperture. The intestine is not a simple cylindrical tube; but 

 its dorsal wall is infolded, forming a longitudinal ridge, the 

 typhlosole (Fig. 85, typh.). This increases the digestive surface. 



The wall of the intestine is composed of five layers: (i) an inner 

 lining of ciliated epithelium, (2) a vascular layer containing many 

 small blood vessels, (3) a thin layer of circular muscle fibers, (4) a 

 layer consisting of a very few longitudinal muscle fibers, and 

 (5) an outer thick coat of chlorogogen cells (Fig. 85, hep.) modified 

 from the coelomic epithelium. The last-named cells also cover the 

 dorsal trunk of the vascular system and extend down into the 

 typhlosole. The function of the chlorogogen cells is not certainly 

 known, but it has been suggested that, since they are present about 

 the alimentary canal, in the typhlosole, and in close proximity 

 to the dorsal blood vessel which carries the food after absorption, 

 they probably aid in the elaboration of food. That they have 

 an excretory function also seems probable, since chlorogogen 

 granules are present in the coelomic fluid of adult worms, make 

 their way through the body wall, especially through the dorsal 

 pores, and pass outside of the body in the mucus (154). They 

 also disintegrate in the coelomic fluid, and the waste products 

 are eliminated through the excretory organs. 



At the sides of the oesophagus are three pairs of calciferous 

 glands (Fig. 87, ces. gl.), one pair in each of the somites from X to 



