Medusae. 61 



confounded with the much humblei* organism, which has Noctiluca for 

 its first name). — Some species of fish spend the early part of their 

 life under the protection of the umbrella of Rhizostoma and Cotylorhiza, 

 and even eat awav parts of the medusaj. 



The mig'i'ations of Medusa3 are of especial interest. At certain per- 

 iods enormous quantities are met Avith in active or passive mig-ration. 

 The shoals of Medusse thus found are so large that ships are often 

 impeded in tlieir 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 Genei-ations — first discovered, in the case of 

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

 Kotzebue expedition round the world — was first formulated as an im- 

 portant biological law by the zoologist Steenstrup. It may be summed 

 up as follows. An individual ^1 produces individuals which are not like 

 itself, but of very different nature, and which Ave 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 Medusse — not of all — this intervening form appears 

 as the so-called 



Hydroid-polypes, 



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

 branches of corals. The Aquarium contains sometimes the very delicate 

 Tubularia, Pennapia, Aglaophenia, and Antennularia (Fig. 112 — 115). 

 Generally they arise from eggs produced by Medusa;, branch by fission 

 and budding and form thus, just as the corals do, larger or smaller 

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

 colony and swim about as Medusfe. These again lay eggs, which give 

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

 the Medusje always remain attached to the colony, and in these cases 

 they are usually so reduced in size and organisation, that chey are scar- 

 cely to be recognized as Medusse 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. 



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 wonder- 

 ful form and beauty of their body is associated with such delicacy, that 

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



