Stkpmens — The Fresh-water Sponges of Ireland. 247 



it is widely ilisLiihuliMl in the nnii-liiiicsLiiiie districts of Llie iiiiiritinie 

 CdUiilii's 111' Ireland, and it eceiirs in Scdlland. 



Dr. Hanitsch, wiio believetl that at least three species with a simihir 

 distribution to the foregoing occurred in Jreland, suggested (21) that three 

 agents might have served to carry geniinules of fresh-water sponges from 

 North America across the Atlantic to the west of Ireland — namely, winds, 

 ocean currents, and birds. At one time it was supposed that strong winds 

 could carry the seeds of plants long distances, but many botanists are now 

 agreed that this means of dispersal has been greatly over-rated, and 

 experiments prove that even seeds provided with special aerostatic apparatus 

 are not carried to great distances (see E. LI. Praeger, Clare Island Survey, 

 Part 10, Phanerogamia and Pterophyta, Proc. lioyal Irish Academy, xxxi, 

 1911). There would be less chance of gemmules being conveyed in this way, 

 as not only are they not provided with wing-like expansions or other 

 structures to enable them to be easily wind-borne, but are, on the contrary, 

 weighted with their armour of siliceous spicules. 



As to ocean currents, it has been suggested that the Gulf Stream might 

 have carried gemmules or entire sponges containing gemmules to this country. 

 It is quite impossible to think that a sponge, such as H. Ryderi, could stand 

 a voyage across the Atlantic Ocean, even if attached to Heating timber, 

 especially when it is remembered that the only forms of this species in which 

 gemmules are abundant are extremely soft and fragile. Nor does it seem 

 probable that separate gemmules should be so conveyed, and this quite 

 apart from the question as to whether they could germinate after prolonged 

 immersion in sea-water. 



With regard to the third agent mentioned by Dr. Hanitsch, it is suggested 

 that birds might convey the gemmules, presumably in mud dried on their 

 feet or feathers, as seeds of plants are known to be sometimes carried. In 

 this connexion I would refer to a paper by Dr. Schartf (37), in which he 

 brings forward evidence from the distribution of various plants and 

 invertebrates (among the latter fresh- water sponges) to support the theory 

 of the pi-esence of a former land-bridge between North America and Europe. 

 Referring to Dr. Hanitsch's statement as to the three possible agents for the 

 dispersal of fresh-water sponges, Dr. Scharff says that he considers the only 

 occasional means of transmission to be thought of seriously is that by birds ; 

 and even in this case he cites evidence to show that birds probably never tiy 

 directly across the Atlantic, nor is there reason to believe that they first set 

 foot on the west coast of Ireland on reaching Europe. 



In addition to the points brought forward by Dr. Scharff, I would suggest 

 the following arguments against the transport of gemmules of 11. Rydit-i liy 



