PORIFERA 



resistance to climatic conditions, powers which must contribute 

 in no small way to the survival of a species exposed to the 

 variable temperatures of fresh water. Thus, if the floating 

 gemmules or the parent skeleton with its attached and dormant 

 offspring should chance to be included in the surface layer of 

 ice during the winter, so far from suffering any evil consequences 

 they appear to benefit by these conditions. Both Potts and 

 Weltner have confirmed the truth of this statement by ex- 

 periments. Weltner succeeded in rearing young from gemmules 

 which had suffered a total exposure of 17 days to a temperature 

 " under 0" C." 



Of important bearing on tlie question of the utility of the 

 gemmules are certain instances in which E. fluviatilis has been 

 recorded as existing in a perennial con- 

 dition.^ The perennial individuals may 

 or may not bear gemmules, which makes 

 it evident that, with the acquisition of the 

 power to survive the winter cold, the prime 

 necessity of forming these bodies vanishes. 

 The perennial specimens are described 

 as exhibiting a diminished vegetative 

 Fig. 73.— Geinimiie of E. activity in winter, the flagellated chambers 

 '£c"''(After Potfso''" ^^^^ ^^ ^^^^nt (Lieberklihn), or present in 

 unusually small numbers (Weltner), the 

 entire canal system may be absent (Metschnikoff), or, on the 

 other hand, it may be complete except for the osculum. 



In tropical countries gemmulation occurs as a defence against 

 the ravages caused by the dry season when the waters recede 

 down their banks, exposing all or most of their sponge inhabit- 

 ants to the direct rays of the sun. The sponges are at once 

 killed, but tlie contained gemmules being thoroughly dried, 

 become efficient distributing agents of the species ; they are light 

 enough to be carried on the wind. It is probable that those 

 individual sponges which escape desiccation survive the dry 

 season without forming gemmules. 



It has been shown experimentally tliat gemmules are not 

 injured by drying — Zykoff found that gemmules kept dry for a 

 period of two years had not lost the power of germination. 



' Weltner, Blatt. Aquar. Fr. vii. 189G, p. 277, and " Spongillidenstudien," 

 Arch. Naturrj. ii. 1893, p. 271. 



