1915J Fauna of the CJiilka Lake : Sponges. 51 



component parts during desiccation, and are finally detached intact by the wind, 

 which wafts them away and, sooner or later, drops them in many cases, on the 

 surface of the water. There they float. We may imagine that a large number are 

 carried by wind or water to quiet nooks among the rocks where they germinate when 

 the floods return, while others are submerged by heavy rain. The majority of these are 

 probably smothered in the mud at the bottom of the lake, but some may fall on 

 stony ground. The masses are rendered extremely light by the spaces between the 

 different layers of horny substance on the surface of the gemmules 1 , and probably 

 some are transported for long distances. The whole mechanism of these structures 

 affords a most interesting example of adaptation on the part of a sponge of recent 

 marine origin, as L. lacustris undoubtedly is, to conditions that can rarely, if ever, 

 occur in the sea. 



Smaller masses of gemmules of the same constitution as the large ones remain 

 embedded in small cavities on the rock on which they were deposited, and their 

 gemmules germinate in situ. This seems to occur mainly at the beginning of the 

 salt-water season, that is to say in November and the beginning of December. At 

 this time of the year I have found many young sponges at different stages of 

 development. In gemmule-masses of the kind, as in the case of Spongilla alba 

 (p. 31, antea), each mass of gemmules produces a single sponge with one osculum. 

 A number of small sponges often arise from different masses deposited close together 

 on a rock or stone. . They do not, however, fuse, when, in the course of growth, they 

 come in contact, but remain distinct, apparently throughout life, although their 

 margins are co-terminous. It is in this way that large areas are often covered with 

 what appears at first sight, but not on careful inspection, to be a uniform layer of 

 sponge. 



Another instance of adaptation to environment is probably to be found in the 

 reproduction of this sponge, viz. in the large irregular cavity which occupies a con- 

 siderable proportion of the interior of the larva (fig. iia). Topsent 2 , discussing 

 the structure of the larva in the different families of Halichondrine sponges, points 

 out that a series of lacunae normally occurs in the primitive epiderm of the embryo 

 and regards these as identical, not merely homologous, with the much larger single 

 cavity found in the larva of Spongillidae. He does not, however, notice that in 

 that larva the cavity is not only of much more regular form but is actually lined by 

 a specialized membrane 6 of which there is apparently no trace in marine types. 

 I have commented tlse where + on the essential resemblance of the Spongillid larva, 



1 Possibly the horny coat of the gemmules of Suberitidae is always deposited in layers; this is 

 clearly the case in Ficulin.i (see Miss Sollas's figure reproduced on p. 230 of Vol. I of the Cambridge 

 Natural History). In most cases, however, it is extremely thin, and I can find no reference in literature 

 to spaces between the layers. 



% Arch. Zool expérim. (5) VII, pp. xiii and xiv (1911). 



3 This is clearly shown in a figure recently published by Nöldeke. Zool. Jahrb. {Anat.) Vlll, 



fig- 1 (io^)- 



4 Journ. As. Soc. Bengal (n. s.) IX, p. 222 (1913)- 



