968 RECORD OF CURRENT RESEARCHES RELATING TO 



The tolerance by Medusae belonging to marine species of fresb 

 water under natural conditions was observed by Mr. H. N. Moseley, 

 in New South Wales, and Professor Agassiz writes : " It strikes me 

 as if the consequences resulting from the finding of the fresh-water 

 Medusa* had been somewhat overdrawn. In the first place, we have 

 two genuine fresh-water Hydroids, Hydra and Cordylophora, and in 

 the second place, as far as my experience goes, it is not conclusive of 

 so fatal an action of fresh water on Meduste as Eomanes would lead 

 us to believe in. We have quite an estuary leading out back of 

 Boston Harbour, extending on the one side to form what we call the 

 back bay, and beyond this up the Charles Eiver as far as Watertown, 

 where there is a dam, about seven miles from the inner extremity of 

 the harbour proper. Here the Charles Eiver falls into this estuary 

 as a fresh- water stream sufficiently large at times to atfect the saltness 

 of the estuary below it at low tide, so that at Cambridge, lialf way 

 from Watertown to Boston, the water is salt only at the highest tides, 

 quite brackish during the first half of the ebb, and comparatively fresh 

 during the last part of the tide. At West Boston Bridge, about one 

 mile from the head of the harbour, the water at the last part of the 

 tide is fresh enough and tastes but little salt. At this bridge there 

 is an abundance of Hydroids which thrive remarkably well on the 

 drainage of the district, and grow to an unusually large size. The 

 species found there which has no free Medusa is Laomedea gigantea ; 

 while of the Hydroids which have free Medusae we find Eucope 

 diapliann, E. pyriformis, and Obelia commissuralis. All these species 

 are, therefore, twice during twenty-four hours exposed to salt water 

 and to nearly fresh water, and thrive remarkably well under the 

 treatment, as must of course theii" free Medusae, which I have caught 

 both at high tide and low water — in salt and in nearly fresh water. 



Other of our Medusae also find their way into this estuary, and I 

 have found in fresh water, at low tide, active '^"^sice, Tiaropsis, and 

 also Aurelice, which seemed unaflected by the large quantity of fresh 

 water in which they were found." 



Origin of the Generative Cells in the Hydroida.t — Dr. Weis- 

 mann, in the paper referred to at page 813 (where the following 

 should have been inserted) gives the results of his examination of 

 Tiibularia mesembryanthemum, Eudendrium ramosum, Gonothyrcea Loveni, 

 Sertularella polyzonias, Plumularia setacea, and Aglaophenia pluma. 

 He confirms the fact that in some cases the eggs of the Hydroida 

 are, without doubt, developed in the endoderm, while the seminal 

 cells are by no means constantly developed from the ectoderm. 

 Thus Eudendrium, Sertularella, and Plumularia have an endo- 

 dermal origin for their sperm-cells, while Tuhularia, Gonothyrcea, 

 Campanularia, Hydradinia, Cordylophora, and Hydra have the origin 

 ectodermal. 



In Hydroids with sessile buds, therefore, we find three combina- 

 tions, out of four which are, in the nature of things, possible ; (a) both 

 sets of sexual products are developed in the endoderm {Hydra, 



* See this Journal, ante, p. 652. f ' Zool. Anzeig.,' iii. (1880) p. 22G. 



