16 Prof. F. Vejdovsky on Freshwater Sponges. 



a peculiar aerostatic apparatus, which is already well known 

 through the older observations of Carter in Spongilla Garteri 

 (" microcell-structure," 'Annals,' 1881, vol. vii. p. 83) and 

 more recently in S. fragilis, but also by the careful investi- 

 gations of Marshall in S. nitens. Dybowski also mentions 

 this layer (" Belegmembran "), which consists of " non-nucle- 

 ate, polygonal cells, from 0'006-0"09 millim. in size." 



This statement is correct, for even the above-mentioned 

 isolated brown gem mules are surrounded by the corresponding 

 envelope ; it is, however, comparatively feebly developed, 

 inasmuch as the cell-spaces, or rather air-chambers, exist only 

 in one layer, and appear like a network, consisting of de- 

 pressed, usually six-sided chamberlets. In the groups of these 

 gemmules this network is already united ; and when several 

 more gemmules come together to form the above-mentioned 

 balls, the air-chambers also increase in number, and the thick- 

 ness of this layer accordingly becomes more considerable. In 

 the spaces between the gemmules especially a thick layer of 

 air-chambers is produced, while the upper poles of the peri- 

 pheral gemmules (/. e. the poles projecting outwards) are 

 covered only by a single layer of chamberlets. 



The coating-spicules certainly appear upon the surface and 

 also scattered in the interior of the air-chamber layer ; but 

 for the most part they group themselves around the horny 

 membranes of the gemmules. The peculiarities of these 

 coating spicules are already sufficiently well known through 

 Dybowski's memoir. 



From what has just been stated it appears that the sponge 

 from the river Danici, which Dr. Dybowski names Spongilla 

 sibirica, agrees essentially with the North-American Spongilla 

 fragilis, Leidy. The more detailed comparison of the two 

 forms shows the following differences and distinctions : — 



1. In Spongilla fragilis, Leidy, the groups of gemmules 

 also occur in fours (var. segregata, Potts) ; but I have also 

 found isolated gemmules, as well as groups of two, three, and 

 up to six. Out of a hundred examples I found no such large 

 number of gemmules in one group as in "S. sibirica." 



2. The horny membrane in S. fragilis, Leidy, is always 

 enveloped by a deep air-chamber layer, in consequence of 

 which the true gemmules seem to be quite concealed, so that 

 they cannot be observed from the surface. In U S. sibirica " 

 the horny membranes are always to be seen distinctly. 



3. The air-tube of Spongilla fragilis, Leidy, possesses walls 

 of the same thickness as those of the gem mule proper. 



