198 Prof. K. Mobius on the Nourishment of 



Before discussing the causes of these changes, I will de- 

 scribe the experiments made with the other and larger aqua- 

 rium. 



Two fifths of the surface of the bottom of this aquarium 

 were covered with a layer of clay, which was laid against one 

 of the narrower walls, and fell with a slope of 12-15° towards 

 the horizontal part of the bottom (fig. 2) . The lower boundary 



Fig. 2. 



Svii'face o£ Waterj 



u 



of this clay-slope was not rectilinear, but curved inwards in 

 the middle. Round this sinuosity the slope was a little more 

 inclined than near the glass walls of the aquarium. It was 

 then filled with sea-water. After this had become quite clear, 

 the bottom was covered with a very thin coat of clay. 



The inclined layer of clay was now carefully covered with 

 unsifted mud from the harbour, inhabited by animals. It 

 formed a slope with an inclination of about 20°. The surface 

 had irregular elevations and depressions, and at its lower 

 margin a reentering curve. 



On the following day the surface had become nearly smooth. 

 Living bivalves and worms projected from it and performed 

 their movements. Scrohicularia alba^ 8. piper ata, and Tel- 

 Una halthica stretched their two mantle-tubes far out of the 

 shell, felt about with the inferior one upon the surface, stirred 

 it up and drew in particles from it ; sometimes a stream of 

 faecal masses passed out of the upper tube and sank down. 

 Here and there a tube of Pectinaria auricoma projected from 

 the mud, and from this also fine mud-masses were sometimes 

 expelled. Leucodore cHiatci waved its filiform tentacles to and 

 fro before its tube. Edwardsia duodecimcirrata spread out its 

 circle of tentacles upon the surface of the mud. Nassa reticu- 



