400 GENERAL BIOLOGY 



body cavity of the earthworm (cited in chap. Ill, p. 178). 

 Ordinarily , nematodes found in such situation are parasites, 

 but here they are found clearing up the lumps of waste 

 chloragogue impedimenta to the worm accumulated in 

 the hinder segments, and the relation seems to be one of 

 mutual advantage. Were the two species mutually 

 dependent in this function, we should call it symbiosis. As 

 it is we call it commensalism, and say that the nematode is a 

 guest, and not a parasite. Commensalism may well have 

 been at times a transition stage in the development of para- 

 sitic habits. 



Study 51. A comparative examination of a series oj 

 parasites of a single order. 



Materials : As good a series of specimens for comparison 

 as may be had in any favorable group ; flowering parasitic 

 plants; copepods, crabs, worms, etc. Hosts may be dis- 

 regarded. 



Compare together as to: 

 Organs of feeding. 

 Organs of reproduction. 

 Organs of locomotion. 

 Organs of sense perception. 



Compare males and females of each parasite if possible as 

 to degree of degeneracy. 



Compare together young larval, and adult forms of the 

 more completely parasitic species selected for study. 



The record of observations should be preserved in notes 

 and sketches. 



j. Pollen Distribution. 



In our study of the green plant series (Chapter III), we 

 have seen how the motile sperm cells of the primeval 

 aquatic plant gradually lost their opportunity for swimming 



