132 On a North- American Freshwater Jellyfish. 



had attached themselves to the glass. We witnessed the 

 spreading of the disk, disclosing, from the first, eight marginal 

 tentacles, a well-defined velum whose aperture was from one 

 third to one fourth the diameter of the disk, and a manubrium 

 depending, about one half the height (or depth) of the bell. 

 Violent pulsating spasms finally resulted in an entire separa- 

 tion from the hydroid and the free life of a roving medusa. 

 I kept no recoi'd of numbers, but it is believed that from 

 twelve to twenty were seen. 



Measurements were difficult, but, as nearly as I could make 

 it out, the jellyfish was at this time about -^ of an inch in 

 diameter. It was of a somewhat prolate dome-shape, and 

 when seen from the polar aspect the manubrium had a clearly 

 quadrate appearance, from whose corners or lobes four radial 

 canals curved downward to the marginal canal. At every 

 point of junction occurred a single tentacle, and another of 

 equal size was found midway between them. These eight 

 tentacles (the only number as yet observed), always pendent, 

 were plentifully charged with thread-cells, and, while suscep- 

 tible of much variation in length, were not seen much longer 

 than one half the diameter of the disk. 



As to temperature, it is obvious that the water of the jars 

 in which this Medusa was developed must have had nearly 

 that of the surrounding atmosphere, with its diurnal changes 

 — say from 60° to 85° at this season j during the winter, in 

 our heated rooms, the temperature is probably more uniform. 

 The hydroid form, in Tacony Creek, being but a few inches 

 below the surface, must be subjected frequently to a tempera- 

 ture at or below the freezing-point. 



Jt is quite improbable that under the present artificial con- 

 ditions any Medusse will attain full maturity this season. It 

 is therefore manifestly unsafe to compare their minute size 

 and general appearance with the totally dissimilar drawings 

 given us of Limnococlium, where the latter had attained a 

 diameter of about ^ an inch. The full life-history of the 

 organism must therefore be again left imperfectly recorded j 

 but 1 am happy to be able to state that my friend Dr. Charles 

 B. Davenport, of Harvard University, has consented to under- 

 take the iurther technical study of it from material we have 

 recently collected, and the drawings &c. left by Dr. Ryder, 

 and to hope that many points now obscure may, through his 

 efforts, be solved. 



To aid the search of others for this — probably the most 

 primitive Coelenterate — it may be well to state that in my 

 experience 1 have only found M. Ryderi in a natural con- 

 dition, living as a messmate among colonies of Bryozoa that 



