12 Prof. A. G. Bourne. On a Hydroid Phase of [Dec. 11, 



cavity in which particles were apparently undergoing digestion. 

 Between the ectoderm and endoderm more or less gelatinous tissue 

 was always developed, separating the two layers. I was prevented 

 from carrying my observations very far, owing to the sudden dis- 

 appearance of all the Medusae from the tank. I therefore propose to 

 reserve further details for a future occasion. It may, however, be 

 useful as a guide for further observations if I state that I ascertained 

 these vesicles to be formed by a nipping off of the genital sac after the 

 liberation of the ripe spermatozoa. I kept these vesicles in isolated 

 vessels and examined them from day to day for four or five days, 

 during which they underwent certain changes, but these changes were 

 somewhat irregular, an irregularity owing possibly to the unfavourable 

 -conditions, possibly to the lateness of the season, all Medusae being 

 about to disappear. 



The mere fact that all the Medusae I could keep long enough under 

 observation threw off all their genital sacs in a similar way, and that 

 these vesicles lived for so long a time without disintegrating, points 

 to this being some regular process of reproduction. 



I repeated part of these observations during last summer, but was 

 unable to continue them, owing to the short period during which the 

 Medusae flourished this year. 



History of Limnocodium to the Present Bate. 



The following table gives the history of Limnocodium up to 

 the present date. For many of these dates I am indebted to Mr. 

 Sowerby. 



March 1879. 

 March 15, 1880. 



June 10, 1880. 



July 31, 1880. 

 Dec, 1880. 



March 17, 1881. 

 June 12, 1881. 

 June 16, 1881. 



June 25, 1881. 

 Aug. 18, 1881. 

 Sept. 30, 188 L. 

 Oct. 15, 1881. 



Pontederia plant introduced into the tank (R.B.S.). 

 Tank (R.B.S.) filled after remaining empty during 

 the winter. 



Adult and very young Medusae observed in im- 

 mense numbers. 

 Medusae had all disappeared. 



Tank (R.B.S.) emptied, damp mud and roots only 



being left. 

 Tank (R.B.S.) filled. 



A few adult Medusae observed; swarms of young. 



About eight adult Medusae, numerous young 

 forms, and some mud, transferred in a jug of 

 water to the Victoria Tank at Kew. 



All Medusae vanished at Regent's Park. 



Adult Medusae swarmed at Kew. 



Kew tank contained adult forms only. 



Kew tank emptied, and a few Medusae only 



seen. 



