NO. 5-] r.iAv.rr/o.v .i.uo. \\; HYDROMEDUSAE. 233 



4, tentacles; 5, otocysts. Other features of subsidiary nature 

 will have mention in their appropriate places. 



It ought to be stated in passing to details and tabulation of 

 results that only in the matter of radial canals and gonads 

 have the entire two thousand specimens been examined, and 

 not all of those with equal detail in each case. In number 

 and variation of tentacles, spurs, anastomosing of canals, etc., 

 the tabulations were limited to one thousand ; and in the case 

 of otocysts to less than one hundred, a reason for which 

 will be given in the appropriate place. I may also state that 

 owing to the insignificant sexual differences, usually requiring 

 microscopic examination to certainly decide, no effort has been 

 made to determine the relation of variation to sex. 



Gonionemus is a rather well-defined trachomedusa, first defi- 

 nitely described from the Atlantic coast by Murbach ('95). It 

 is characterized by the typical four radial canals, which have 

 the folded gonads suspended along their under surfaces, cruci- 

 form manubrium, as seen in transverse section, normally with 

 four oral lobes and sinuously folded lips. Tentacles are 

 numerous and similar, each characterized by a suctorial bulb 

 near the tip, beyond which it often makes a sharp bend. 



These medusae abound from June to October in a small pond 

 near the Marine Laboratory, known as the "eel pond," which 

 communicates with the open harbor by a narrow inlet. Lately 

 they have been taken in the outer harbor, though in small 

 numbers. I may mention this fact of the localized habitat, 

 since it may well be a question whether its peculiarity may not 

 be an important factor in the physiological aspects of the varia- 

 tions to be considered. 



Taking up now the consideration of the several points in the 

 order given, attention will first be directed to the 



Radial Canals. 



On this point an estimation of the ratio of numerical varia- 

 tion based upon fifteen hundred specimens gave nearly five 

 per cent (4.82). On the matter of their form or disposition, 

 i.e., whether in their course to the marginal canal the bells 



