228 



COLLEGE ZOOLOGY 



blastula cells, called mesomeres. These cells divide, forming two 

 mesoblastic bands which later become the epithelial lining of 

 the ccelom. The embryo escapes from the cocoon as a small 

 worm in about two or three weeks. 



Behavior. EXTERNAL STIMULI. The external stimuli that 

 have been most frequently employed in studying the behavior 

 of earthworms are those dealing with thig- 

 motropism, chemotropism, and phototropism. 



THIGMOTROPISM. Mechanical stimula- 

 tion, if continuous and not too strong, calls 

 forth a positive reaction ; the worms live 



FIG. 159. A, the anterior segments of two copulating earthworms. Slime 

 tubes encircle the pair from the 8th to the 33d segment. B, cocoon, freshly 

 deposited, of an earthworm, surrounded by one-half of a slime tube. (After 

 Foot, in Journ. Morph.) 



where their bodies come in contact with solid objects; they 

 apparently liijje to feel the walls of their burrows against their 

 bodies, or, when outside of their burrows, to lie or crawl upon the 

 ground. Reactions to sounds are not due to the presence of 

 a sense of hearing, but to the contact stimuli produced by vibra- 

 tions. Darwin showed that musical tones produced no response, 

 but that the worms contained in a flower-pot drew back into 

 their burrows immediately when a note was struck, if the pot 

 were placed upon a piano, this result being due to vibrations. 



CHEMOTROPISM. In certain cases chemotropic reactions 

 result in bringing the animal into regions of favorable food con- 

 ditions, or turning it away from unpleasant substances. Mois- 

 ture, which is necessary for respiration, and consequently for the 

 life of the earthworm, causes a positive reaction, provided it 



