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of a number of smaller tubes, each of which contains an open- 

 ing which leads into the common hollow or stomach. From 

 the rim of the cup, which is often zigzagged, there hang long 

 filaments, which the animal can extend or contract at will. They 

 are frequently of great length and when the medusa swims 

 float gracefully behind. They are furnished with numerous nettle- 

 cells such as we saw in the actiniae. They are equally a 

 capital protection for the delicate bodies of the medusae. The 

 disagreable burning sensation we often experience while ba- 

 thing in the sea , proceeds often from our having come into 

 contact with a medusa, and the effect is sufficient to kill small 

 organisms. Some oceanic medusae , which attain a size of 30 

 to 60 centimetres, and a weight of 50 to 60 pounds, are even 

 dangerous to man. The rowing of the medusae is a very in- 

 teresting fact. At certain periods enormous quantities of these 

 creatures collect together and commence their travels ; ships 

 are often delayed for hours and even days by meeting these 

 swarms. The animals are so close together that a stick thrust 

 among them remains in an upright position, as if stack in the 

 mud, and ordinary rowing-boats can scarcely force a passage. 

 Such emigrations are probably the result of over-population by 

 these creatures of a certain ocean-district, and consequent want 

 of food ; perhaps also of climatic influences ; the small troops 

 that not rarely appear on the coasts and in the bays, are, in 

 all probability, connected with the peculiar process of propa- 

 gation observed in many medusae and known as alternation-of- 

 generation. 



This process, first discovered by the German poet Adalbert 

 von Chamisso, while studying salpae during a cruise round 

 the world, has since been enounced as an important scientific 

 theory by the Danish naturalist Steenstrup. It consist chiefly 

 in the following changes: an animal which we will call A pro- 

 pagates, but the young it produces do not resemble their pa- 

 rent at all, but are quite different animals which we will call 

 B\ B propagates, and again its young are not like itself, but 

 like A. In other words: in order that A can propagate A, an 

 intermediate number B is necessary. 



In the case of many medusae not in all this intermediate 

 member appears in the form of a creature scientifically known 

 under the name of a Hydroid-polype, which looks exactly like 

 a plant, very similar to a coral stock. The aquarium sometimes 

 contains the Tubularia, Campanularia. and Sertularia, 

 all representatives of the Hydroid-polypes. They come from the 



