62 Part second. 



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

 foot-note). 



The Siphonophora are looked upon by most naturalists as freeswim- 

 ming colonies. It is not rare to find individuals of one species thus uni- 

 ted; 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 indepen- 

 dently of the other ; but the colonies are made up of very differently 

 shaped (polymorph) individuals, each form undertaking one of the diffe- 

 rent functions , which have to be performed. Special nutritive polypes 

 undertake the nutrition of the colony; special bell-shaped individuals, 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 this difference, that 

 among them the polymorph individuals (the workers , drones, queen) are 

 separate one from the other , whereas in the Siphonophora they are in- 

 separably united While Physophora (Fig. 86) 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 bearing on its upper sur- 

 face a triaugular crest , which catches the wind like a sail and enables 

 the colony to drift along on te surface of the water. 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 Siphcnophora and Medusas in the 

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

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

 sparent 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. Pro- 

 bably 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 animals *) , even the delicate Cte- 



*) They 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 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 them together in large numbers, and it is only necessary 

 to steer into these Correnti to catch them in swarms. The large species are carefully scoo- 

 ped up whith buckets and transferred to the Aquarium ; the smaller ones and the microscopic 

 ones are caught with a net made of the finest silkgauze , 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 can 

 therefore not be taken out of the water. They only live a few days in the Aquarium , although 

 they are placed separately in large glass cylinders and seem to flourish very well for a time. 

 Most pelagic animals shine at night, some with a very strong light, and contribute largely to 

 the Phosphorescence so frequently. 



