268 



General Biology 



that a coelom is defined as the cavity lying- between the digestive tract 

 and the outer body wall. 



There are in the earthworm, 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, 



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

 br., brain; cr., crop; fu., 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 



intestine 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, they may aid in some digestive 

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



