26 BULLETIN OF THE BUREAU OF FISHERIES. 



an important part of the food of tish fry, the relations of tisli and medusaa in certain 

 cases may be due to the relation of both to a common source of food. 



In this connection may lie noted another feature of medusoid life, namely, suscep- 

 tibility to certain parasites. When working upon the regeneration of medusa', I 

 found several species of Protozoa very closely associated with them and. under the 

 limitations of the aquaria, often exceedingly troublesome, seriously interfering with 

 the progress of the experiments. This suggested the probability of a parasitic 

 relation. I have also found several species of Hydromedusse infested with a minute 

 nematode, a species of Distonrttm, which in many cases was so abun'dant as to per 

 meate almost every part of the medusa. The parasitism of a small aetinian, a species 

 of F.il irn rth'itt, on Mia in iopsis liiih/i, is well known, though the entire history of the 

 case is not yet fully determined. 



The irregular and apparently capricious occurrence and distribution of medusa' 

 has long been known hut little understood. That many have a seasonal period there 

 can hardly be reasonable doubt. Such for example is true of Hybocodon, Sa?'xia, 

 Thim. I'i niiiir'iii, etc.. among Hydromedusa'. and Dactylometra, Cyanea, and to a less 

 extent Aurelia, among Seyphomedusa\ Further consideration will he given to this 

 in connection with the subsequent descriptions. 



Agassiz has called attention to the gregarious habit noted among many medusa' 

 at certain times and places (cf. Catalogue of North American Acalephae, pp. 4i'. 46), 

 and has sought to explain the phenomenon as due to breeding conditions. This 

 seems to me a somewhat gratuitous assumption, as facts do not confirm it so far as 

 my observations have gone. As I have elsewhere suggested, it seems much more 

 probable that these aggregations may be quite as easily accounted for by the influence 

 of prevailing winds, tides, etc. At no time have 1 seen Cyanea in greater numbers 

 than in April, when the specimens were very small, hardly more than 1' to '■'< inches 

 in diameter, and sexually immature. A change in the direction of wind or tide 

 would disperse them again within a day's time so that only by careful search could 

 an isolated specimen here and there he noted. This is true likewise of many Hydro- 

 medusa', which may often occur in large numbers for a day or two and then as 

 suddenly disappear. The occurrence of some other species, however, is less easily 

 explained. For example, Rliegmatodes tenuis and Stauroplwra laciniata I have taken 

 but twice at this station in ten years. Both of these being of littoral habitat their 

 appearance could hardly be accounted for by a heavy wind from the direction of the 

 open sea. as may be the case with pelagic forms like Traohyriema or Physalia. 

 When we know more definitely the details of the lift 1 history of such forms, light 

 will undoubtedly he thrown upon this as well as many other more or less obscure 

 phenomena concerning them. 



A feature more or less similar to those just discussed is the apparently nocturnal 

 habits of not a few medusa'. Whether the occurrence at the surface during the 

 early evening or night can be said to constitute a definitely nocturnal habit may be 

 an open question, but certain it is that there is here a fairly well-defined responsive- 

 ness to light and darkness. This has been experimentally demonstrated in several 

 well-known cases — for example. Gonionemus, Pennaria, etc., where advantage has 

 been taken of it to secure 1 the discharge of ova at times other than those normal to 

 the species. May there not be here, also an explanation of a fact that has been often 



