62 Part second. 



especially in calm weather, specimens of Physophora (Pig. 89), Forskalia 

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

 rium, is due to the particular method employed in their capture 'see 

 foot-note). 



The Siphonophora are looked upon by most naturalists as free- 

 swimming colonies. It is not rare to find individuals of one species thus 

 united; the corals form one of the most striking examples. The case of 

 the Siphonophora, however, is somewhat different; here the individuals 

 are not all similar and performing similar functions, so that each can 

 live independently of the other; but the colonies are made up of very 

 differently shaped (polymorph) individuals, each form undertaking one of 

 the' different functions, which have to be performed. Special nutritive 

 polypes undertake the nutrition of the colony; special bell-shaped indi- 

 viduals, like medusae, perform the swimming movements; true medusae 

 are charged with the reproduction. In short, we have a "division of 

 labour*' taking place as it does among the ants and bees; but with 

 the difference, that among them the polymorph individuals (workers, 

 drones, queen) are separate one from the other, whereas in the Siphono- 

 phora they are inseparably united. -- While Physophora (Fig. 89) gives the 

 impression of medusae bound together by a slender thread or stalk, in 

 the beautiful blue Sallee Man, Velella (Fig. 91), this common stalk is 

 replaced by a wide horny disk overshadowing all the polypes and bear- 

 ing on its upper surface a triangular crest, which catches the wind like 

 a sail and enables the colony to drift along on the surface of the water. 

 Physophora appears clearly to be a group of animals, Velella only one, 

 but there are all transitions in the degree of union. 



CTENOPHORA. 



The Ctenophora agree with the Siphonophora and Medusae in the 

 transparency of their body, a character which is of frequent occurrence 

 in marine animals. There are some transparent Mollusks, there are 

 transparent Annelids, transparent Crustacea and even transparent Fishes. 

 The reason for this occurrence of transparent forms must be sought for 

 in the advantage which they gain by such a character. Probably the 

 advantage consists in the difficulty which their enemies have in seeing 

 them, and in the facility with which they can surprise and capture their 

 prey. These transparent forms*), even the delicate Ctenophora and 



*) These transparent forms live chiefly in the open sea and are therefore 

 often grouped together under the name of Pelagic Animals. In a calm sea, and 

 if the light be not too intense, they are found at the surface of the water; other 

 conditions cause them to sink to some depth. They can thus not be caught 

 regularly, and in stormy weather they may be missing in the Aquarium for some 

 time. Usually the currents at sea bring them together in large numbers, and it 

 is only necessary to steer into these "Correnti" to catch them in swarms. The 

 large forms are carefully scooped up with buckets, and transferred to the Aquarium; 

 the smaller ones and the microscopic ones are caught with a net made of the finest 

 eilkAgauze, in the meshes of which they become entangled, and often of course 

 damaged. Most Ctenophora and Siphonophora, in spite of their size, are so 

 delicate that they collapse immediately they are handled the least roughly, and 



