REPEODUCTION AND HEREDITY 259 



of life. In the flattened margin of the shell are numerous small 

 cavities which during the dry period are filled with air and act 

 like swim-bladders, keeping the buds suspended on the surface, 

 when they once more reach the water. Winds and currents then 

 drive the statoblasts over vast stretches of water and finally each 

 produces a young Bryozoon. 



One may frequently see that asexual reproduction regularly 

 alternates with sexual, so that one asexual generation, or a 

 limited number of asexual generations, is always succeeded by a 

 sexual generation. In that case we speak of an alternation of 

 generations. As long ago as 1815 this phenomenon was dis- 

 covered in one of the locomotor tunicates, the Salpa, by the poet- 

 naturalist Chamisso. Fig. 61 shows in the foreground a large speci- 

 men of the asexual generation of Salpa democratica, the ' nurse,' 

 which always leads a hermit life. At the posterior end we see the 

 Stolo prolifer from which sprout successively several colonies of 

 Salpae. As soon as these buds have reached a certain size they 

 separate from the body of the ' nurse ' and float about, joined 

 together in long chains. These chain-Salpae are sex-animalcules 

 which possess sexual organs, but no stolon. Soon after its separa- 

 tion from the stolon each individual of this chain produces an 

 ovum, and afterwards male spermotozoa. They are therefore 

 hermaphrodites. Fertilization and embryonic development take 

 place in the interior of the maternal body, and from each ovum 

 there develops finally a solitary Salpa, the ' nurse,' which once 

 more proceeds to the formation of chain- Salpse. This completes 

 the cycle of development. As the body of most Salpae possesses 

 a high degree of phosphorescence, it is a wonderful sight to see, 

 from a seat in a boat, these long glowing chains move like fiery 

 serpents below the surface of the water. 



The alternation of generations is most interesting in numerous 

 members of the jelly-fishes. Here it is frequently connected with 

 a strict division of labour and a resulting great diversity of forms. 

 While the fresh-water polyp Hydra is a sexual animalcule and an 

 asexual generation in one individual, numbers of its near and 

 distant relatives have abandoned the faculty of producing repro- 

 ductive cells, and proceed to the formation of special sexual 

 individuals, the Medusae. 



During prolonged north winds the ports of the Baltic Sea 



