Stephens — The Fresh-water Sponges of Ireland. 241 



The shorter lieniimilc-spicules have a straight shaft set with usually one 

 to several strong, straight spines. Sometimes the shaft projects for a short 

 distance above the disc. The terminal discs are toothed, the indentations 

 being deeper than in the corresponding American fonn. The length of these 

 amphidiscs is 0'03-00o5 mm. ; the diameter of the disc is 0-02 mm. 



Gemrnules occur in great numbers in the typical form of H. liydcri; and 

 they have been found inatuve as early in the year as June. When mature 

 they are a bright yellow colour. Their diameter varies from 05 mm. to 

 0-7 mm. 



■ ■ Group II. 



Hetcromcijcnia Ri/deri Potts, foim pictovcnsis Potts, or Lalce Form. 



This form grows in lakes, and is very compact and hard to the touch. Jt 

 corresponds to the form, at first named H. pictoreiisis, which was discovered 

 in lakes in Nova Scotia. 



The sponge is pale yellowish or greyish-white in colour. It is circular 

 in outline, and is usually not more than 20 or 30 mm. in diameter, but 

 sometimes reaches a diameter of 50 or 60 mm. The surface is even, but 

 under the lens is seen to be raised up into minute points by the tips of the 

 main skeleton-fibres, which penetrate the dermis, and project very slightly. 

 The sponge is thickest in the middle, and especially in the larger specimens 

 is sometimes raised up into knob-like elevations. The oscula are about 

 1 mm. in diameter, but are rendered more conspicuous by the fact 

 that immediately below the dermis numerous fiuTows radiate from them 

 in all directions. In the autumn the sponge begins to die away at the 

 centre, so that many specimens are found in the form of a fiat ring, the 

 centre of the sponge having completely decayed away. 



The skeleton is arranged in the same way as in the typical form. The 

 main fibres, which are about 0'025-0'05 mm. in thickness, are a spicule- 

 length apart, and are therefore more closely placed than in the typical form, 

 as the spicules in the lake form are shorter. In the interior of the sponge 

 the skeleton is very confused, but becomes more regular towards the surface. 



The skeleton-spicules show great variation. The shaft is straight or 

 slightly curved, and terminates at each end in a longer or shorter point, or 

 one or both ends may be rounded off. It is densely covered with. 

 shai'p spines throughout its entire length. Sometimes the spines are 

 scattered more sparsely along the middle of the shaft, and are crowded 

 towards the ends. The smaller the spines the more thickly are they placed. 

 Some spicules are set with comparatively few very strong spines. Some 

 specimens possess fairly uniform spicules, others very varying ones, but 



