BEING A WORM 



193 



and the circular muscles contract, while a shortening of the worm 

 results from a contraction of the longitudinal muscles and a relaxing 

 of the circular muscles. Each stiff seta is placed in a little sac, 

 from which it extends out beyond the surface of the body. Inside 

 the sac, attached to the seta and to the outer body wall, are two pairs 



endocterra ^ 

 •muscle.-, 



peritoneum ^^^ 

 TOphridium 



^Cuticle ectoderm Circtxlar 



^."peritoneum. 



muscle 



^nephricCiTopore 

 <-Seta 



/ verztro-l vessel 

 lataml vessel 



'wentral rjerve- cord. 

 subnsLcral vess-©! 



Cross section through earthworm. Compare this with cross section of Hydra. 

 What advances in complexity of structure flo you find:' In the earthworm the 

 most noticeable difl'erence is seen in the coelom. which is formed by a sphtting of 

 the mesodermal bands in the embryo (seen on page 197). Note that the coeiom 

 is completely lined by a delicate membrane, the peritoneum. Notice also the 

 longitudinal fold or typhlosole which gives more surface to the inner wall of the 

 intestine. What is its function .^ In the diagram, the funnels of the nephridia 

 are not shown. Explain why this is so. 



of muscles by means of which the seta can be directed forwards or 

 backwards, depending on the direction the worm is traveling. When 

 the worm is moving forward, the anterior end is extended, the setae, 

 that are pointed backward, are set into the ground, serving as an- 

 chors, while the posterior end of the worm is pulled forward by means 

 of the contraction of the longitudinal muscles. 



