230 



METAZOAN PHYLA 



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XXVI-( 



XXXE- 



Opemng^ 

 ofovicfucf 



^Openln0 of 

 vas deferens 



Se/n/naf 

 groove 



^ Cl/fe//un7 



xxxvn- 



end of the body. From one end of the animal to the other the alimentary 

 canal extends, a tube within a tube. It is held in place by the partitions, 

 the coelomic spaces thus becoming ringlike. In the ventral portions of 

 metameres IX to XV are the reproductive organs, while above the ali- 

 mentary canal is the dorsal blood vessel and 

 below it the ventral blood vessel and nerve 

 cord. Each metamere, except the first three 

 and the last, also contains a pair of nephridia. 

 The coelomic spaces are lined by a delicate 

 epithelium known as the 'peritoneum. They 

 are filled with the coelomic fluid, which is 

 colorless and which, when the worm contracts, 

 can flow from one space to another through 

 a small opening in each septum above the 

 ventral nerA'e cord. 



269. Alimentary Canal and Metabolism. — 

 The alimentary canal is divided into a greater 

 number of regions than in any type previously 

 discussed and these are more highly special- 

 ized (Fig. 136). These regions include a buccal 

 cavity; a muscular pharyiix; a narrow esophagus; 

 a thin-walled ciliated crop, or proventriculus; 

 a thick-walled muscular gizzard; and a thin- 

 walled intestine, which begins at metamere XIX. 

 The food of earthworms consists of organic 

 matter in the soil and of living or decaying 

 leaves. The worms will also eat bits from the 

 bodies of dead animals when they can secure 

 them. The organic matter in the soil may be 

 gathered at any time, but the fragments of 

 leaves are secured from the surface of the 

 ground at night. Taken in at the mouth by a 

 sucking action of the pharynx and mixed with 

 a secretion from salivary glands in the pharyn- 

 geal wall, this food is passed to the crop. 

 From here it goes into the gizzard where it is 

 ground and is then passed on into the intestine 

 where digestion and absorption take place. 

 Feces are egested at the anal opening. They appear in the form of castings 

 outside the burrows. Enzymes are produced in the intestine which act 

 on proteins, fats, and carbohydrates. 



270. Circulatory System. — The circulatory system of the earthworm 

 is a complicated system of blood vessels through which the blood is 

 forced by a sort of peristaltic action of the muscle fibers in the wafls of 



'Sefae 



-Anus 



Fig. 134. — An earth- 

 worm, Lumhricus terrestris 

 Linnaeus. Ventral side of 

 the more anterior and the 

 most posterior segments. 

 From a preserved specimen. 

 X Ij-^ in length and about 

 2 in diameter. Segments 

 numbered in roman. 



