PHYLUM ANNELIDA 219 



tions. Septa are absent between somites I and II, and incom- 

 plete between somites III and IV, and XVII and XVIII. The 

 walls of the coelom are lined with an epithelium, termed the 

 peritoneum. The coelomic cavity is filled with a colorless fluid 

 which flows from one compartment to another when the body 

 of the worm contracts. In somites IX to XVI are the repro- 

 ductive organs (Fig. 158); running along the upper surface of 

 the alimentary canal is the dorsal blood-vessel (Fig. 153, dors. v); 

 and just beneath it lie the ventral blood-vessels {vent, v) and 

 nerve-cord {n.co). 



Detailed Anatomy and Physiology. -- Digestion. — The 

 alimentary canal (Fig. 154) consists of (1) 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 {oes), 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 muscular- walled gizzard 

 (giz) in somites XVII and XVIII, and (6) a thin-walled in- 

 testine {hit) extending from somite XIX to the anal aperture. 

 The intestine is not a simple cylindrical tube; but its dorsal 

 wall is infolded, forming an internal longitudinal ridge, the 

 typhlosole (Fig. 153, typh). This increases the digestive surface. 

 Surrounding the alimentary canal and dorsal blood-vessel is a 

 layer of chlorogogen cells (Fig. 153, hep). The functions of these 

 cells are not known with certainty, but they probably aid in the 

 elaboration of food and are excretory. Three pairs of calciferous 

 glands lie at the sides of the oesophagus (Fig. 154, oes. gl) in seg- 

 ments X to XII ; they produce carbonate of lime, which prob- 

 ably neutralizes acid foods. 



The food of the earthworm consists principally of pieces of 

 leaves and other vegetation, particles of animal matter, and soil. 

 This material is gathered at night. At this time the worms are 

 active; they crawl out into the air, and, holding fast to the tops 

 of their burrows with their tails, explore the neighborhood. 

 Food particles are drawn into the buccal cavity by suction pro- 



