ON THE HETERONEREIS STAGE OF NEREIS 

 KOBIENSIS McINTOSH. 1 



AARON L. TREADWELL. 



Among a collection of polychaetous annelids made by the U. 

 S. F. C. steamer Albatross in the Hawaiian Islands in 1902, and 

 sent me for study, were large numbers of a heteronereid which 

 by means of the general character of the head, the presence of a 

 peculiar hook-shaped seta in the anterior parapodia, and especi- 

 ally from the form of the paragnathi, I have identified as the 

 above-mentioned species. 



Externally the bodies of both male and female individuals 

 show very clearly the distinction between " modified " posterior, 

 and " unmodified " anterior somites characteristic of this sexual 

 phase of Nereis, the modifications following in general the usual 

 direction, /. e., a broadening and flattening of all parapodial lobes 

 and a replacement of the ordinary form of seta by one with a 

 very broad, flat, terminal joint. A constant external sex differ- 

 ence is found in the male in the dorsal cirri of somites 27. 

 Beginning with somite 2 the dorsal cirrus is larger than in the 

 female, and on successive somites as far as the seventh there is a 

 gradual increase in the size of this organ, until on the seventh it 

 is very prominent, composed of a thick, cylindrical basal portion; 

 with a broad flattened tip, ending in an acute point. This con- 

 dition is similar to that of the form described by.Verrill under the 

 name of Nectonereis. 1 On the eighth somite there is an abrupt 

 change to the ordinary form of cirrus. 



Internally, extensive modifications appear. On a surface view 

 of a mature female, one sees that the whole body, anterior, " un- 

 modified " as well as posterior " modified " region is crowded with 

 eggs, seeming not to be isolated in somites, but packed together 

 in a continuous cavity. That the transverse septae are actually 



1 Published by permission of Hon. George M. Bowers, United States Commis- 

 sioner of Fisheries. 



2 Verrill, A. E., "Invertebrates of Vineyard Sound," Bull. U. S. F. C., p. 591, 

 1872. 



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