200 



ELEMENTARY ZOOLOGY. 



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ture and manner of living. The 

 most of them are transparent and 

 are therefore beautiful objects of 

 study under the microscope. Their 

 setae are better developed than 

 those of the earthworm, and their 

 nervous system and sensory organs 

 are often better developed also. 

 Many have spots of pigment known 

 as eyes. Dero, Fig. 58, is one of 

 these worms. 



217. Marine Annelids. Many of 

 the segmented worms live in the 

 ocean. These are rather more 

 highly developed in many ways 

 than the earthworms. They have 

 better organs of locomotion, having 

 many bristles on definite leg-like 

 projections of the body wall. The 

 head region is also more specialized, 

 often having eyes, tentacles, and 



FIG. 58. Dero, a fresh-water oligochsetous annelid, 

 in optical section. Enlarged about 30 times. Trace 

 out structures already lettered in Figs. 56 and 57. 

 oe, oesophagus; p, pavilion lined with ciliated ento- 

 derm ; pr, outgrowth from the anal segment ; z, a zone 

 in front of anal segment in which new segments are 

 being formed ; z', an old segment in the middle of the 

 body that is developing a division of the worm into 

 two. The anterior half of 2' will produce a region 

 like 2 for the anterior individual. The posterior half 

 of 2' will produce a head and four segments like 1-4 of 

 the parent worm. 



Questions on the Figure. Trace the 

 differentiations of the digestive tract. 

 Follow carefully just what must take place 

 in the dividing segment (z') in this process 

 of forming two worms. 



