CHAPTER XX 



WORMS 



Vocabulary 



Anterior, the end toward the head, usually the end that precedes 



in locomotion. 



Posterior, the end farthest from the head. 

 Analogous, having similar function. 

 Homologous, having similar structure or origin. 

 Setae, hair-like projections by which some worms travel. 

 Incalculable, impossible to estimate. 

 Degeneration, loss of ability to perform function, loss of structures 



due to disuse. 



The worms may be taken as a class of animals showing a mod- 

 erate degree of specialization. They include the common earth- 

 worm, leeches, bloodsuckers, tapeworms, horsehair worms, etc. 



THE EARTHWORM 



External Features. The earthworm is familiar and will do to 

 represent the group. Its slender body is divided into rings or seg- 

 ments. The larger end, near which is a light colored girdle, is the 

 head (anterior) end; while the vent, or opening of the intestine 

 marks the opposite (posterior) extremity. Projecting from each 

 segment are four pairs of bristles (setae) which are operated by 

 separate muscles and are used in locomotion. The girdle secretes 

 the case in which the eggs are deposited and near it are the tiny 

 openings of the egg and sperm ducts, since the organs of both 

 sexes are found in the same animal. On opening the body, the wall 

 is found to consist of a very thin cuticle and two thick layers of 

 muscle, one running lengthwise, and the other around the body. 



Digestive System. Inside the body wall, the large digestive 

 system can easily be recognized, there being a muscular pharynx, a 



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