THE EARTHWORM 



26? 



coelom is defined as the cavity lying between the digestive tract and the 

 outer body wall. 



There are muscles, nerves, glands, connective tissue, blood-vessels, 

 epithelium, and endothelium, just as in the frog, though not developed 

 as elaborately. There is also a delicate lifeless coat called the cuticle. 



THE DIGESTIVE SYSTEM 



The alimentary canal (Fig. 166) begins at the anterior end with a 

 mouth cavity or buccal pouch, extending from the first to the third 

 somite inclusively, the thick muscular pharynx ( ) 



lies in somites four and five ; the oesophagus, a narrow straight tube, 



B 



Fig. 166. 



A, Longitudinal vertical section through the anterior portion of an earthworm. 

 br., brain ; cr., crop ; /tt., seminal funnel ; giz., gizzard ; int., intestine ; n.c., nerve 

 cord ; neph., nephridia ; oes., oesophagus ; oes. gl., oesophageal gland ; ph., pharynx. 



(From Parker and Haswell after Marshall and Hurst). 



B, Section of the Alimentary Canal, c, chlorogogen cells : cm, circular muscles ; 

 ep, epithelium, lining the canal; Im, longitudinal muscles; v, blood vessels.' (From 

 Conn, modified from Sedgwick and Wilson). 



extends through the sixth to the fourteenth somite ; a thick muscular- 

 walled gizzard in somites seventeen and eighteen ; and a thin-walled 

 intestine from somite nineteen to the anal opening. 



The dorsal wall of the intestine is folded in, forming a longitudinal 

 ridge, called the typhlosole ( ). This gives the in- 



testine considerable expansion and affords additional surface for diges- 

 tion. 



The wall of the intestine, as in the frog, is composed of five layers, 

 (Fig. 166, B): 



(1) 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, 



(5) An outer thick coat of chlorogogen cells ( > 

 modified from ,the coelomic epithelium. 



It is supposed that, because these chlorogogen cells lie in the typhlo- 

 sole close to the dorsal blood vessel that they may aid in some digestive 

 process. Then, because chlorogogen granules are present in the coelomic 

 fluid of adult worms and make their way to the outer part of the body 



