DISPERSAL BY STATOBLASTS 187 



perish by the way, but this lavish ness of material to 

 ensure dispersal is nothing strange to the economy of 

 Nature. 



The formation of gemmules is not peculiar to our 

 British freshwater sponges, but characterises the group 

 all over the world. So far, only three or four exceptions 

 are known to the rule, and these are met with in species 

 which are somewhat recent immigrants into fresh water. 

 In the tropics, where aestivation generally takes the place 

 of hibernation, the gemmules are formed on the approach 

 of the dry season. 



The freshwater Hydra is the only member of the 

 Hydrozoa which is at all generally distributed throughout 



FIG. 59. Statoblasts of a PolyzDan (Plumatella fruticosa, 

 Allm.). On the left the free form with its surrounding 

 float ; on the righb the sessile form. Magnified 65 

 diameters. After Vosseler, from Lampert, "Das Lebeii 

 der Binnengewasser." 



our freshwater area, and this, in marked contrast to its 

 marine relatives, does not pass through a free swimming 

 stage ; the egg is surrounded by a horny case (Fig. 

 58), and the young Hydra hatches out in the adult 

 form. 



The same avoidance of a free larval stage is manifested 

 by the freshwater Polyzoa, the " statoblasts," which serve 

 as seeds, reminding us in many respects of the gemmules 

 of freshwater sponges : like these, some are secured to 

 the parental skeleton, and others are sent floating on the 

 stream to take what fortune they may find (Fig. 59). 

 In two species (Cristatella mucedo and Pectinatella 

 magnifica) the floating statoblasts are provided not only 



