Prof. F. Vejdovsky on Freshwater Sponges. 17 



The polar air-tube of Spongilla fragilis plays an important 

 part in the existence of the gemraules. It is in direct con- 

 nexion with the low superior process of the gemmule, which 

 is generally regarded as an aperture of issue for the young 

 sponge enclosed in the chitinous membrane. Whether it is 

 really an aperture, however, I cannot state with certainty, as 

 I never succeeded, either in living gemmules or in longitudinal 

 sections, in demonstrating anything of the kind. It rather 

 appears to be much more probable that the polar process is 

 also completely closed by the horny membrane, by which 

 the space within the gemmule appears to be completely cut 

 off from the air-tube. The air-tubes of the dry gemmules of 

 Spongilla fragilis are occupied by large air-bladders. A 

 similar apparatus has been demonstrated by Carter and Mar- 

 shall in S. Cartert, and I can confirm the statements of those 

 naturalists from my own investigations. Moreover I am 

 acquainted with similar arrangements in Tabella reticulata. 

 Cart., in which the air-tube is inconsiderably inflated. The 

 apparatus in question is very interesting in the remarkable 

 North-American genus Carterius. In this it forms a high 

 hollow tube, which, when the gemmules are thrown into 

 water, is always directed upwards. The gemmules in this 

 genus, however, are not globular, but only slightly convex 

 above, while below they are hemispherical, so that the median 

 longitudinal section through a gemmule resembles a little 

 boat, the air-tube rising in the centre appearing not unlike a 

 mast. The air-bubble enclosed within it evidently assists 

 not a little in maintaining the gemmule at the surface of the 

 water. In Spongilla fragilis the air-tubes are of compara- 

 tively more considerable dimensions than in any other species 

 that I have examined ; they must, however, contain a larger 

 quantity of air, in order to sustain the certainly heavier groups 

 of gemmules for a time at the surface of the water. 



The memoir by Wilhelm Hetzer (' Die deutschen Siiss- 

 wasserschwamme,' Inaugural Dissertation, Tubingen, 1883), 

 which probably appeared simultaneously with my previous 

 work, contains descriptions of some freshwater sponges which 

 had been already established by Noll (' Zoologische Garten,' 

 1870), but which require a fresh investigation in order to 

 determine their rank and their relations to those described in 

 my monograph. This applies especially to Spongilla Lieber- 

 kuhniij Noll, and also to S. contecta } Noll, which, in my 

 opinion, must agree with S. fragilis. Whether Spongilla 



Ann. (& Mag. N. Hist. Ser. 5. Vol. xv. 2 



