Si'EPMKNS — The Fresh-water Sponges of Ireland. 243 



more lobiiaL ronus ol' the species. In specimens wiLli very slender spicules 

 the shafts of the amphidiscs are smootli. The sliafts of tliicker amphidiscs 

 are I'urnislied with dhc or mure spines. 



Gemmules are usually i'airly numerous. They measure about 0"5-0'7 mm. 

 in diameter, and thus are as large as those in the stronger forms of tlic 

 species. It may be noted that gemmules are scarcest in the most robust 

 form {2nctovensis, Potts). 



If. Ityderi, form Baleni, is merely a starved form of the species. It 

 occurs iu very small quantities in the lakes and streams in wliich it is 

 found. 



The spicules of the specimens found in Church Lough, Inishbofiii, agree 

 in every particular with those of the sponge from Sable Island, which was 

 named II. Macouni MacKay (26). The measurements of the spicules froDi 

 these widely separated islands are of interest. In the Sable Island sponge 

 the oxea are 0-I5-0-26mm. long, with a maximum diametei' of O'OOomm. 

 The longer amphidiscs are 0-035-0'05 mm. long ; the shorter, O-Ol 8-0026 mm. 

 long. In the Inishbofin sponge the oxea are 0'16-0"24 mm. long, with a 

 maximum diameter of 0-005 mm. The longer amphidiscs are 0-035-0-04 mm. 

 long; the shorter, 0-025-0'03 mm. long. 



These extreme forms are not sharply divided from the form referred to 

 on page 244, which occurs fairly abundantly in certain lakes. Specimens 

 have been collected which show every link between the two. 



Although the lake and river forms differ so much from one another, all 

 the intermediate links between them can be obtained by collecting the 

 sponge in a lake where it grows abundantly, and then tracing it down the 

 course of the stream which drains the lake. This has already been described 

 in the case of a lake, Lugaloughauu, in Co. Mayo (41). Since that aeeouni 

 was written many other localities have been searched, always with similar 

 results, namely, that at a varying distance below a lake, usually just below 

 it, the hard lake form of H. Rydcri dies out and the soft, lobed, river form 

 takes its place. In several instances specimens of H. Rydcri, apparently like 

 the lake form, were found at some little distance down the river. Hard, 

 compact specimens were taken several hundred yards down a rapid stream, 

 flowing from Lough Unshin iu Co. Donegal (41, p. 14); but they dift'ered 

 from the lake specimens in being much larger, and in possessing matiirfe 

 gemmules. Their spicules also had begun to change. In the lake the 

 skeleton-spicules were straight, and their ends were usually rounded oil'. 

 They measured 125-0-175 mm. by O'Ola mm. In the specimens from the 

 stream all the spicules had pointed ends, and many of them were slightly 

 curved. They were longer and more slender, measuring 01o;")-0-25 nun. by 



