242 Proceedings of the Royal Irish Academy. 



usnally one or other type of spicule predominates in a specimen, and all the 

 specimens from a given lake have the same tj-pes of spicules.^ The spicules 

 also vary very much in both length and thickness. They usually measure 

 from 0"12-0"25 mm. in lenj^th. The variation is not so great in a single 

 sponge; and in many the maximum length is 0"2mm. The maxinnim thick- 

 ness is connuonly 0'015 mm., but may be as much as 00- mm. In specimens 

 with very robust spicules there often occur very short thick spicules, 

 measuring about 0'05-0-08 mm. by 0'02 mm., or even 0-025 mm. 



The gemmule-spicules are the same as in tlie typical form, but are some- 

 times more robust. The gemmules are very scarce, and very many specimens 

 may be collected from neighbouring lakes in the autumn without finding a 

 single gemmule. When present, the mature gemmules are bright yellow, 

 and are tlie same size as those of tlie typical form. 



Embryos are often present in the lake form in great numbere. 



Gitour III. 

 Hfleromeijenia Ryderi Potts, form Balcni Potts. 



Tliis form usually occurs in small, lobed masses on water-plants. It is 

 very soft to the touch, and of a jwlc yellowish colour. More rarely it grows 

 in small, soft, more or less cii-cular, whitish-grey dims on the under-surface 

 of stones. In the former state it resembles the typical H. Mydcri; in the 

 latter, it approaches the pidovfiisis form in external appearance. ' 



The skeleton is arranged on the same plan as in the preceding forms ; 

 but, owing to the extreme sienderncss of the skeleton-spicules, at first sight 

 it appears to differ considerably. The main fibres, which are about 002- 

 0*03 nun. in thickness, run upwards in an irregular manner through the 

 sponge, dividing occasionally. They consist of multiserially arranged spicules, 

 and are united by single spicules, or by bundles of spicules, which do not 

 form continuous fibre-s. Other oxea lie scattered irregularly through the 

 sponge. The whole arrangement of the skeleton is often rather confused. 



The skeleton-spicules are straight or slightly curved oxea, which taper 

 evenly to sharp points. The shaft, except at the ends, is thickly covered 

 with very fine spines. The spiuation cannot be made out on the thinnest 

 spicules. The oxea are about 0'16-0"26 mm. in length. The maximum 

 diameter is about 0005 mm. ; but most of the spicules are much finer. 



Both kinds of gemmule-spicules are exceedingly slender. The terminal, 

 recurved spines of the longer are usually straighter than in the typical 

 form, and the discs of the shorter ones are deeply indented. Thus the 

 difference between the two kinds of amphidi.scs is less marked than in the 



