556 



Moist 



EVOLUTION OF ANIMALS 



SUMMER WINTER 



Dry 



Part V 



feet 





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Fig. 28.3. Seasonal locations of earthworms. Summer: worms feed, mate and 

 crawl about on the ground on moist nights; leave castings on the surface; burrow 

 in the upper soil, the depth depending on moisture; cluster in dry soil. Winter: 

 migrate below the frost line and hibernate; conserve moisture and heat by clus- 

 tering. (Courtesy, Morgan: Animals in Winter. New York, G. P. Putnam's Sons, 

 1939.) 



cells, of the dorsal pores from the body cavity to the outside, and above the 

 sensory cells (Fig. 28.5). These and the outer openings of the nephridia 

 (kidneys) are invisible except by microscopic examination. Other passage- 

 ways are those of the two oviducts on the fourteenth segment; four minute 

 openings of the seminal receptacles in the furrows between segments 9 and 

 10, and 10 and 11; and the sperm ducts on segment 15 (Fig. 28.11). The 

 surface of the body is covered with layers of iridescent cuticle secreted by the 

 outer cells of the skin. Cuticle and mucus compose the trail left on the side- 

 walks after a night's wandering. Earthworms are sensitive to touch especially 

 at the ends of the body. Each contact cell has a hairlike tip that projects 



