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 habitualty 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. Birger (91, p. 650) has sufficiently shown the fallacy of any 
comparison with Eubostrichus, which resembles Nectonema at most in 
