General remarks. 



57 



> Now, the class of swimming and floating animals which are 

 quite free from the bottom consists of two divisions with regard 

 to their capabilities of locomotion, and these two may be compared 

 to the past and present types of airships. Like the older balloons 

 many animals float freely in their medium, the specific weight 

 of which approximates to their own. These passively follow the 

 movements of the medium. The swimmers, however, are com- 

 parable to the Zeppelins, fitted with strong motors (muscles) 

 and rudders, which overcome the sinking due to their own weight 

 and the resistance of the medium, alter the direction of their 

 movement at will and in point of speed compete successfully 

 with the swiftest expresses. Between these two extremes there 

 are all immaginable transitions and the more an animal approaches 

 the condition of complete lack of active movement, and the less 

 the importance of its own powers of swimming against the passive 

 change of position by means of water currents, so much the more 

 closely does it approach that group of marine animals (and plants) 

 which are distinguished as Plankton. — The visitor will find 

 typical sessile animals in tanks 4, 8, 9, 21, and 22 and tank 20 

 contains some of the larger floating animals, which are distinguished, 

 like many Plankton organisms, by their great transparency. 



According to the different means of locomotion just described, 

 the animals choose out their habitats in the sea. The floating 

 and permanently swimming animals live between the bottom and 

 the surface of the water. They possess a number of different con- 

 trivances to overcome the tendency to sink, contrivances which 

 consist either in an approximation of their specific weight to 

 that of the water or of a slowing down of the sinking through an 

 increase of the frictional resistance. The crawling, occasionally 

 swimming and fixed animals populate, on the other hand, the 

 coastal slopes and the rocky shallows (called "Secche") rising up 

 from the mud. On these grounds they settle on the green meadows 

 of Posidonia (sea-grass), between luxuriant bushes of sea-weed 

 and on the red incrustations of calcareous algae. Others live 

 among the remains (detritus) of dead organisms, under stones, 

 in the sand and on top of and in the mud which cover the floor 

 of the Gulf. 



These are the hunting grounds from which the Aquarium is 

 replenished. 



Two chief forms of net are used. Firstly ground-nets, which 

 are dragged over the bottom and either tear away the animals 

 which live between or on the stones by means of their heavy 

 and sharp-edged frames or else graze lightly over the ground 

 and stir up the more moveable forms, driving them into the sack 

 of the net. Secondly floating nets made of silk gauze, which 

 are drawn slowly through the water. To the ground-nets are 

 attached untwined pieces of rope in which many animals become 



