NEREID AE. 193 



spoken of as the " heteronereis " stage. Heteronereis was erected as a genus 

 by Oersted (Annulatorum Danicorum conspectus, 1843, p. 19) for epitokous 

 forms of Nereis under the assumption that they were independent of any other 

 known forms. In the metamorphosis into the heteronereis form the eyes increase 

 in size, often to a marked degree, the prostomium may become indistinguishable 

 though sometimes becoming otherwise modified, and the palpi commonly undergo 

 a more or less pronounced reduction. At the same time the body becomes 

 marked off into two regions differing strongly in the appearance of the parapodia 

 of the two parts. Of these the anterior, or so-called nereid division, which is 

 comparatively short, retains parapodia of the ordinary form, while the posterior 

 or heteronereid division has parapodia conspicuously modified. These com- 

 monly bear special foliaceous lobes and numerous, large, special, natatory setae. 

 The sexual products arise in the posterior division where they may remain, 

 giving it a darker and more opaque appearance contrasting with the commonly 

 colorless and often transparent anterior region, or the products in other cases 

 crowd forward into all the somites. Between the epitokous male and female 

 a sexual dimorphism exists which is often very striking. Differences between 

 the two forms are noticeable, particularly in the tentacles, palpi, and cirri, and 

 sometimes in the prostomium (C/. Platynereis polyscalma, sp. nov. p. 219). 

 The genus Eunereis of Malmgren, which has proved valid, was based upon the . 

 epitokous form of E. longissima Johnston ; and Naumachius Kinberg and Hedyle 

 Malmgren were apparently based upon the epitokous forms of species of Pseudo- 

 nereis. The real relationship of the heteronereis was first established by Malm- 

 gren (Zeitschr. wiss. zool., 1869, 19, p. 466) in Nereis pelagica. One and the 

 same species may exhibit both a smaller, active, pelagic epitokous form and a 

 larger, sedentary heteronereis which remains in a tube on the bottom, as has been 

 well estabhshed e.g., in Nereis dumerili (Cf. Westinghausen, Mitth. Zool. stat. 

 Neapel, 1891, 10; Mesnil and Caullery, Ann. Univ. Lyon, 1898, p. 146, ff.). 

 It may also happen that a species may at one time reach sexual maturity without 

 undergoing any metamorphosis, while at other times in passing into the epitokous 

 stage it undergoes the change into the heteronereis. Viviparity, as an occasional 

 but probably not necessary phenomenon, has been established for some species, 

 such as the hermaphroditic (proterandrous) Nereis diversicolor. 



The nereids are essentially Uttoral animals, occurring mostly between the 

 tide-marks or at but moderate depths, though sometimes found down as deep 

 as 1,525 fms. {e.g., Nereis longisetis). They commonly construct tubes in 

 the fissures in rocks or under stones, in the mud in the eel-grass stretches, and in 



