NEREIS AND HETERONEREIS 



277 



ducts as to be functionless, and the tail develops special sensory 

 papillae. 1 



In the parapodia an increase in size and a sharper delineation 

 of the various parts take place ; then flattened foliaceous out- 

 growths (Fig. 147, x.y} arise from certain lobes of the feet, in which, 

 too, the blood supply becomes greatly increased. The old chaetae 

 are pushed out by the development of new ones of quite a different 

 shape ; these are jointed like the old ones, but the appendix is, 

 in many species at least, flattened and oar-shaped (Fig. 123, C, p. 

 246) ; and the chaetae are arranged in a fan-like manner. Both 

 these modifications are in evident 

 relation to the free -swimming 

 habit which the Heteronereid now 

 adopts. The new foot serves as a 

 swimming organ, the old one was 

 a walking appendage. 



Whilst some species, such as 

 the common British N. diversi- 

 color, undergo no change, and 

 others become modified as just 

 described, others, again, are poly- 

 morphic. Claparede was the first 

 to show that JV. dumeriiii may 

 occur in at least five different 

 mature forms; these differ from 

 one another in size, colour, mode 

 of life, character of the eggs, etc. 



The immature forms may become 



ripe and lay eggs while still re- 

 taining the "Nereid" characteristics, or these immature forma 

 may become " Heteronereids " 2 whilst the sexual elements are 

 ripening. There are then three different kinds of males and 

 of females in this one species, some being found at the bottom 

 of the sea, as the large Heteronereid form, while the small 

 Heteronereid swims on the surface. The relations of these 

 various forms to one another, and the causes lending to the 



3 Claparede, " Anm'-lides Clu'-topodes du Golfe de Naples," Supplement, 1870; 

 and Wistinghausen, ML Zool. Stat. Nettpcl, x. 1891, p. 41. 



2 Claparede used the term "epigainous" for this phase; Ehiers employed the 

 term "epitokous," whilst he called the "Nereid" phase "atokous,"' under the im- 

 pression that the worm did not become mature in this condition. 



d, neuropodial cirrus ; ./.-, y, foliaceous 



