105 



antero-external angle of the plates and their free ends point backwards 

 and inwards , thus corresponding with the position of the lamellae 

 in the embryo of Scorpio and in Spiders. 



Thus, while accepting Macleod's theory in its main lines, I 

 would suggest that the lung-books of the Arachnids were probably de- 

 rived from a series of paired platelike appendages , not united in the 

 middle line , by a gradual fusion of their edges with the abdominal 

 walls of the body. 



6. Arrangement of keeping alive fresh water animals. 



By Dr. J. Dewitz, Berlin. 



eingeg. 5. Januar 1892. 



Wherever expensive arrangements of biological insitutions are not 

 at our disposal it is rather troublesome to keep fresh water animals 

 alive, as the water intended for certain kinds of animals is to be daily 

 renewed. Hence it would be advantageous to find out a method of 

 preserving fresh water animals without water. 



I observed for biological purposes Paludina vivipara, a pond snail, 

 common in the North Eastern part of Germany. It is easy to obtain 

 this kind of snail in great quantities in summer and spring in Berlin, 

 the Spree and the canals connecting with that river teeming with the 

 animals mentioned, even in the centre of the city. Still in the late 

 fall and winter all water-animals dip through the mud of the ground 

 and the snails in question can be caught only with great difficulty. 

 This fall now I have collected a large quantity (about 350) of Palu- 

 dina and preserved them as follows : 



An appropriate piece of canvas or a small dampened towel having 

 been spred over the bottom and sides of a plate, the snails are put into 

 the plate so as to cover the bottom and sides of the vessel. The mol- 

 luscs thus arranged form a thin layer and are covered with an other 

 wet towel and placed in a cold room. Care has to be taken that the 

 water does not stagnate. For this purpose the towels are to be wrung 

 out. Every fourth day the snails are to be washed, the dead ones to be 

 removed and the towels to be washed and wrung out again. 



This arrangement retains the snails in a permanently damp con- 

 dition and I have preserved the collected animals, about 350 in num- 

 ber, for three months and I believe that I can keep them alive all 

 through the winter. Still I have to add here that three snails on the 

 avarage die weekly and it seems that the young molluscs being more 

 sensitive to this proceeding than the adult snails show a greater mor- 

 tality than the latter. Not all species of snails, however, can stand 

 equally well the mentioned treatment. Bithynia tentaculata^ f. i., was 



