182 BULLETIN OF THE 



5. Parasitic Nature. 



Nectonema possesses neither eye spots nor sense organs, such as are 

 present in practically all cases of free-living, and especially of pelagic 

 forms. The general structure of the alimentary tract, its diminutive 

 size as compared with that of the animal, its occasional closure anteriorly, 

 the complete degeneration of its posterior portion, and the absence of 

 any functional anus, speak even more strongly against the possibility of 

 regarding Nectonema as primarily a free form, and practically force one 

 to the conclusion that it is a parasite, which passes its larval life in 

 some unknown animal, wandering out of its host at sexual maturity 

 and passing the final stage of its life history in a free condition, in which 

 alone it is at present known. On the analogy of Gordius, the host may 

 be surmised to be some fish or crustacean, and, since Nectonema is not 

 so rare as has been supposed, it ought not to be difficult in its proper 

 home to discover its host. 



There are certain facts which should be mentioned in this connection. 

 As has already been said, Nectonema was caught only on an ebbing tide 

 and in the bay near shore, not in open water. And although a large 

 amount of truly pelagic material was obtained in the same manner, yet 

 numerous Annelids which are by no means truly pelagic were found in 

 the same towing. The latter form part of the bay or shore fauna which 

 in towing near the land is habitually found in the net. In the same way, 

 Nectonema, which is probably set free from some one or more of the 

 small fish or Crustacea which inhabit the shores of the bays or shallow 

 water in general, will live normally in the little coves and quiet places 

 along shore, but may be carried out by the tidal currents even to some 

 distance. It is probably found at or near the surface at night only, and 

 at the bottom during the day. The greater prominence of the contrac- 

 tile portion of the muscular layer in the male would seem to indicate 

 that it is the more active of the two, and to this may be due in part 

 the much larger number of males captured. 



VII. Comparison with other Forms. 



Numerous possible relationships have been suggested for Nectonema, 

 many of which rest upon resemblances of a superficial character, such as 

 the comparison with Sagitta on the ground that both possess lateral 

 fins. Biirger ('91, p. G50) has sufficiently shown the fallacy of any 

 comparison with Eubostrichus, which resembles Nectonema at most in 



