127 



discovered process known as heterogamy. It is characterised bv the 

 succession of differently organized sexual generations living under 

 different nutritive conditions. 



Heterogamy, which was first discovered in certain small Nematodes 

 (Rhabdonema nigrovenosum and Leptodzra appendiculata), can scarcely 

 be explained otherwise than as an adaptation to changed conditions. 

 For when the embryo is developed as a parasite in conditions favour- 

 able for the acquisition of nutriment, it gives rise to a sexual form 

 so different in size and structure from that which arises if the 



B. 



FIG. 114. A, Rhabdonema nigrovenosum of about 35 mm. in length at the stage when the 

 male organs are ripe. G, genital gland; O, mouth ; D, alimentary canal ; A, anus ; A, 

 nerve ring; Drz, gland cells ; Z, isolated spermatozoa. B, Male and female Rhabditis, 

 length from about 1 '5 to 2 mm. ; Ov, ovary; T, testis; F, female genital opening; Sp, 



spicula. 



embryo leads a free existence in damp earth or dirty water (i.e., in 

 conditions not so favourable for the acquisition of nutriment), that 

 we should, from the difference in their structure, place the two forms 

 in different genera. Rhabdonema nigrovenosum from the lungs of 

 Batrachians and the free-living Rhabditis follow each other in a 

 strictly alternating manner (fig. 114, A, B). Other cases of hetero- 

 gamy are afforded by the Bark-lice (Chermes), and the Root-lice 



