POLYCHAETE WORMS, PART 1 149 



a shorter anterior region with the normal type of setae and a more 

 posterior larger region in which the segments are compressed and 

 flattened anteroposteriorly like an accordion, with extra development 

 of thin foliaceous lamellar plates on the parapodial lobes (fig. 44c), 

 the normal setae being replaced by a special type of compound homo- 

 gomph swimming setae in which the blades are flattened, paddlelike 

 (fig. 44e); with or without a third most posterior region or "tail," 

 with the usual type of parapodia. In the males, the anterior dorsal 

 and ventral cirri are often modified and clubbed, with curved and 

 pointed tips (fig. 43A). In the females, the cirri may be slightly 

 modified. In the modified region in the males, the dorsal cirri are 

 denticulate or crenulate on their lower side and the anal segment is 

 furnished with long to short papillae (so-called anal rosette) . 



The transformation into the heteronereis is often accompanied by 

 extensive histolysis of certain muscles of the body waU and of the 

 digestive tube and by reorganization of the musculature. The 

 heteronereids may swim to the surface in swarms. All degi'ees of 

 variation in reproductive pattern are found in the group, as indicated 

 in the follomng (for more details, see systematic discussion; also see 

 Reish, 1957, and Clark, 1961): 



(1) Reproduction in the atokous stage, without epitolvous or heter- 

 onereis stages, as (a) Nereis arenaceodonta (includes N. caudata), 

 males and females pair up in tube for lengthy period, where eggs are 

 laid, fertilized, and then incubated by the male ; (b) Nereis diversicolor, 

 males much less numerous than females ; they show a type of pseudo- 

 copulation where numerous females move to a mature male and 

 surround him, when the eggs and sperm are given out, followed by 

 fertilization and development in the tube ; (c) Nereis limnicola Johnson 

 (includes N. lighti Hartman; see Smith, 1958, 1959), an internal 

 self-fertilizing hermaphrodite; viviparous, producing young of about 

 20 segments. 



(2) Reproduction by male and female swarmers, forming only 

 slightly modified epitokes but not distinct heteronereids, the adults 

 swarming on the flats or at the surface, as Nereis virens and Nereis 

 iaponica. 



(3) Reproduction by markedly modified heteronereids; the adults 

 abandon their tubes or burrows and swarm at the surface, the males 

 pursuing the females. This may be followed by: external fertili- 

 zation, where the sexual elements are emptied directly into the water 

 and where fertilization and development ensue, as in Nereis succinea, 

 Nereis pelagica, Platynereis dumerilii (at Naples); or internal fertili- 

 zation by a unique copulatory mechanism, as in Platynereis dumerilii 

 megalops in the Woods Hole region. Both males and females die after 

 spawning and the ova give rise to planktogenic larvae. 



