62 Part second. 



especially in calm weather, specimens of Physophora (Fig-. 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 nor perform similar functions, so that each can live 

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

 differently shaped (polymorph) indiA^iduals, 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 medusge 

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

 labour » taking place as it does among the ants and bees ; l)ut with 

 this difference, that among them the polymorph individuals (the 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 stiff 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 Avater. 

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

 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 transparency among animals 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 tlieir 

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



*) They live chiefly in the open .sea and are therefore often grouped together under 

 the name of PeJagic Aiiunals. In a cahu 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 not thus be caught regularly, and in stormy weather they may be missing from 

 the Aquarium for some time. Usually the currents at sea bring t^eni together in large 

 numbers, and it is only necessary to steer into these "Correnti „ to catch them in swarms. 

 The large species are carefully scooped up with buckets and transferred to the Aquarium; 

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

 gauze, in the meshes of which they be 'ome entangled, and often of course damaged. — 

 Most Ctenophoia and Siphonophora, in spile of their size, are so delicate that they col- 



