Tanks No. 20 and 21. 
23 
The migrations of the Medusae are of especial 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 way through. These migrations are yet 
to be explained; they may be the mere over-popu¬ 
lation of tracts of water which happened to be in 
motion. The lesser swarms which are sometimes 
met with on the coasts and in bays are in all pro¬ 
bability due to the 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. 65), by the poet 
Adalbert von Chamisso, on the Kotzebue expedition 
round the world — was first formulated as an important 
biological law by the zoologist Steenstrup. It may 
be summed up as follows. An individual A pro¬ 
duces 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 Medusae — not of all — this 
intervening form appears as the so-called 
Hydroid-polypes, 
Carmarina 
hastata, 1 /g nat.size. 
Tank 20. 
Fig. 24. Tubularia larynx. Fig. 25. Pennaria Cavolinii. Tank 21. 
Tank 21. 
which have entirely the appearance of plants and are very similar to 
branches of corals. The Aquarium contains sometimes (in tank 21) the 
very delicate Tubularia, Pennaria, Aglaophenia, and Antennularia 
(Fig. 24—27). They arise from eggs produced by Medusae, branch by 
