Mousse. 61 



under the protection of the umbrella of Bhizostoma and (Jotylorhiza, and 

 even eat away parts of the ineduse. 



The migrations of Medusae are of especial interest. At certain per- 

 iods enormous quantities are met with in active or passive migration. 

 The shoals of Medusae thus found are so large that ships are often im- 

 peded in their course for days together , the animals swimming in so 

 dense a mass that a stick, plunged into their midst, remains upright as 

 if driven into something viscid, and ordinary rowing boats can scarcely 

 force their way through. These migrations are yet to be explained. The 

 lesser swarms which are sometimes met with on the coasts and in bays 

 are in all probability due to a curious mode of reproduction of these 

 animals known as alternation of generations. 



This alternation of Generations first discovered, in the case of the 

 Salpae (see p. 86), by the poet Adalbert von Chamisso, on the Kotzebue 

 expedition round the world was first formulated as an important bio- 

 logical law by the zoologist Steenstrup. It may be summed up as follows. 

 An individual A produces individuals which are not like itself, but of 

 very different nature, and which we may call B. B also gives rise to 

 individuals unlike itself, but like A. In other words: for A to reproduce 

 A forms, an intervening form B is necessary. In the case of many Me- 

 dusae not of all this intervening form appears as the so-called 



Hydroid-Polypes, 



which have entirely the appearance of plants and are very similar to 

 branches of corals. The Aquarium contains sometimes the very delicate 

 Tubularia, Pennaria, Aglaophenia, and Antennularia (Figs. 112 

 115). Generally they arise from eggs produced by Medusae , branch by 

 fission and budding and form thus, just as the corals do. larger of smaller 

 colonies. At fixed periods they produce buds which separate from the 

 colony and swim about as Medusas. These again lay eggs , which giv 

 rise to new Polypes. But this is not the case in all species. In some the 

 Medusas always remain attached to the colony , and in these cases they 

 are usually so reduced in size and organisation , that they are scarcely 

 to be recognized as Medusae at all. 



The Hydroid-polypes are found in enormous masses on stones, reefs 

 and rocky coasts among the sea-weeds. The animals , which form these 

 colonies, live on the smallest Crustacea, worms, infusoria, etc., wnich come 

 within reach of their tentacles and are stunned by the action of their 

 stinging-cells. 



SIPHONOPHORA. 



These, among the most wonderful of the inhabitants of the sea, are 

 at once the delight and the despair of the naturalist. For the wonderful 

 form and beaut}' of their body is associated with such delicacy, that it 

 breaks to pieces at the slightest touch. The fact that, nevertheless, espe- 

 cially in calm weather, specimens of Physophora (Fig. 89), Forskalia 

 (Fig. 92), Hippopodius (Fig. 90) and others may be seen in the Aqua- 



