Medusa; or Jelly-Fish. 5q 



Some kinds of fish spend the early part of their life unmolested 

 and apparently protected beneath the umbrella of Rhizostoma 

 or Cotylorhiza, while other Medusae eat fish after having first 

 attacked and wounded them with their batteries of stinging-cells. 



The migrations of Medusae are of special interest. At certain 

 periods enormous quantities are met with in active or passive 

 migration. The shoals of Medusae thus found are so large that 

 ships are often impeded 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 wa}^ through. 

 These migrations are yet to be explained. 



While many Medusae develop directly from eggs in the 

 usual way, others reproduce themselves by a complicated and 

 peculiar process known ?iS Alternation of generations which was first 

 discovered by the poet Adalbert von Chamisso (see p. loi) and first 

 established as an important biological law by the zoologist Steen- 

 strup. It may be summed up as follows. An individual A produces 

 individuals which are not like itself, but of quite a different nature, 

 and which we may call B. B also gives rise to individuals unlike 

 itself, but hke A. In other words: for A to reproduce yl-forms, 

 an intervening form B is necessary. In the case of many Medusae — 

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



Hydroid-Polypes, 



which have entirely the appearance of plants and are essentially 

 similar to branches of corals. Generally they rise from eggs pro- 

 duced by Medusae, branch by fission and budding and thus, form 

 just as the corals do, larger or smaller colonies. At fixed periods 

 they produce buds which separate from the colony and swim 

 about as Medusae. These again lay eggs, which give rise to new 

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

 Medusae always remain attached to the colony, and then 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., which come within reach of their tentacles and are stunned 

 by the action of their stinging-cells. 



In the Aquarium they are represented by the very delicate 

 Aglaophenia, Antennularia, Tubularia and Pennaria (figs.iii — 114). 

 Many of these colonies go through a stage of degeneration, during 

 which the polyps disappear from the branches, which are reduced 

 to stumps; after some months of latent life, however, the colonies 

 regenerate again. 



