342 A MONOGRAPH OF THE 



reproduction is by internal geinmulation, while in Spongilla 

 the same vital function is always exercised through the 

 medium of ovaria ; and in these organs a peculiar structure 

 and class of spicula prevail which are never found in the 

 reproductive organs of any of the species of the marine 

 genus Isodictya. 



Although Dr. Johnston adopted the two British species 

 as described by Dr. Fleming, he still retained doubts as to 

 their being in reality more than one ; and it was not until 

 I had made careful microscopical examinations of the ovaria 

 of each that their distinctive specific characters were deter- 

 mined to my own satisfaction. 



2. Spongilla lacustris, Johnston. 



Ephi'Datia. Canalium, Fleming. 

 Halichondria lacustris, Fleming. 



Sponge. Sessile, branching ; surface more or less hispid. 

 Oscula simple, dispersed, small, and numerous. Pores 

 inconspicuous. Dermal and interstitial membranes 

 pellucid, spiculous ; spicula numerous, fusiformi-ace- 

 rate, entirely spined ; spines abundant, conical, acute. 

 Skeleton. Spicula subfusiformi-acerate. Ovaria sub- 

 globose ; spicula acerate, much and variably curved, 

 disposed at right angles to lines radiating from the 

 centre of the ovaries, entirely spined ; spines conical, 

 acute. 



Colour. Dark green. 



Habitat. Lakes and rivers of England and Scotland. 



Examined. In the living state. 



Much uncertainty has existed regarding the specific 

 distinctions belonging to the two well-known European 

 species of Spongilla ; and I can only attribute this inde- 

 cision on the part of naturalists to their having hitherto 

 appealed to the characters of external form and substance 



