136 The Irish Naturalist. December, 



lie, is mostly covered with upland pasture and peat bogs, 

 and the water is typical of such a district, its character 

 being determined by the absence of limestone in any 

 quantity, and by the presence of non-calcareous rocks and 

 peat. The water is soft and slightly acid, the hydrogen 

 ion concentration (pH) being 6-8. The temperature of 

 the water during the previous twelve months had ranged 

 from 0° to 23^ C. 



The most conspicuous inhabitant of the filter was the 

 gymnoblastic hydroid Cordylophora lacustris Allman, the 

 stalks of which covered the walls of the compartments. 

 This genus and species was first described by Allman^ 

 from material collected on an old submerged boat in the 

 Grand Canal Docks, Dublin, in 1842. It does not appear 

 to have been recorded subsequently from Ireland. It has 

 been since found in various parts of Great Britain, in 

 Belgium, Holland, Germany, Sweden and the United States. 

 This species occurs most commonly and luxurianth^ in 

 brackish w^ater. It was suggested by Prof. Carl Semper~ 

 that it had originally been an inhabitant of estuaries and 

 the mouths of rivers. " Since that time, 1854, the animal 

 has in many places migrated into rivers." There seems 

 little evidence to support this statement, and it is now 

 known to occur at localities far distant from the sea. This 

 species, the only one of its group living in fresh water, 

 would naturally be fovmd most frequently near the sea, 

 where it would attract the attention of students of the 

 Hydro zoa. In fresh water, it shuns the open, it is 

 inconspicuous, and so is easily overlooked. Kraepelin^ 

 records C. lacustris as an important constituent of the 

 fauna found in the pipes of the water-supply of Hamburg. 

 In the year 1890, De Vries * also observed it in large 

 quantities in the pipes carrying the water-supply of 

 Rotterdam. 



1 Proc. R. Irish Academy, 1 844, vol. ii, p. 395 ; Phil. Trans. Roy. 

 Soc, 1853, p. 367. 



2 Animal Life, p. 152. 



^ " Die Fauna der Hamburger Wasserleitung." Abh. Naturwiss, 

 Ver. Hamburg, ix., 1885. 



* " Die Pflanzen und Thiere in den dunkeln Raumen der Rotter- 

 dammer Wasserleitung." Bericht iib. d. Biolog. Unters. d. Crenothrix- 

 Commission zu Rotterdam vom Jahre 1887. 



