248 ProcectUngs of the Roi/al Irish Acndemij. 



birds, fioin a consideration of tlie liabitat and mode of growth of that 

 species : — 



(1) //. /i//<7(/v" does not grow where i,here is mud, but in clear water on 

 tlie stony beaches of lakes, or on the siouy beds of rivers, so that there would 

 be little or no material to cement the gemmules to the feet or feathers of 

 liirds. 



(2) Gemmules are extremely scarce in the lake form of II. Iti/deri in 

 North America (Potts and MacKay). In Ireland they are so scarce that 

 inuuheds of specimens may be collected even late in the year without finding 

 a single one. 



(3) Gemmules ai-e numerous in the river form of U. Ityderi in North 

 America (I'ott«). In Ireland (and ? in North America) they are most 

 abundant in specimens in rapidly flowing clear rivers and streams, with 

 boulder-strewn beds, at a short ilisUmcc below a lake (see p. 219). The 

 possibility of gemmules becoming attacheil to birds under these conditions 

 would seem to be slight. 



The chances of a .successful introduction of the species into Ireland by 

 means of bird.s would bo lessened by the fact that JI. Jiijdcri does not grow 

 in this country in lakes or rivere ou the limestone, so that a bird carrying 

 gemmules would have to deposit them, if they were to geiniinate success- 

 fully, in fresh water in a non-limestone district; and as the sponge grows 

 with difliculty in lakes with boggy shores, they would have to be deposited 

 in clear wattT on a stony bottom. Tiierefoie 1 would consider that the 

 distribution of II. llijdcri, aa at present known, cannot be explained by any 

 of these occasional means of disiiereal ; but that it may be cited among the 

 evidences of a former land connexion between North America and Europe. 



Dr. Annandale (3, p. 11) refere to this (juestion of the dispersal of fresh- 

 water sponges. In discussing the relationships of the fresh-water sponges 

 and polyzoa of the Malabar Zone of India with those of Africa and of tlie 

 countries east of India, he mentions aerial currents (in this case the monsoon) 

 and njarinc ourrenU as possible agents in the dispcreal of these animals. 

 ISut he dismisses both in a few words, as the resting reproductive bodies of 

 t,he genem in the area.s under consideration are either fixed to some 

 solid "support, or are without a special apparatus to render them light. 

 Ur. Anuandale states that the most satisfactory explanation as yet put 

 forward to account for the relationships of these or other groups of animals 

 is that of a former land connexion between the countries involved, that is to 

 say, lietwcen Africa and the Malaysia through Malabar, in, pcrhai»s, late 

 Cretaceous times. 



