4 G. 0. Sårs. 



For it might be supposed that these forms have been 

 accidentally transferred to the aquaria with resting ova adhering 

 to the aquatic plants, which it is generally necessary to in- 

 troduce into the aquaria for the sake of the aeration of 

 the water. It is impossible to deny, that in reality such a 

 transfer might easily take place, and thus give occasion to 

 a misapprehension of the origin of the species appearing in 

 the aquaria. I find it therefore necessary here to state ex- 

 pressly, that I have been well aware of this circumstance, 

 and have accordingly taken precautionary measures. For 

 instance, all the plants used for this purpose have been pre- 

 viously carefully cleansed from mud, and moreover washed 

 several times in pure water, in order to remove any adhering 

 particle. They are also, as a rule, taken from places where 

 Entomostraca do not exist in any number, and at times when 

 resting eggs are not developed. Notwithstanding these pre- 

 cautions, I am still somewhat uncertain about the origin of 

 2 small Lynceids found in my aquaria. The one is the 

 ubiquitous form Chydorus sphæricus (Muller), the other a 

 small species of the genus Alona, A. rectangula G. O. Sars. 

 Both these species occur almost everywhere, even in the 

 shallowest pieces of water, and thus the possibility of an 

 accidental transfer, either direct or indirect, is not quite ex- 

 cluded. With regard to all the other species, on the other 

 hand, I am quite assured that they are derived from the 

 mud with which the aquaria were prepared. 



The present paper is devoted only to the Cladocera, 

 in a succeding paper I intend to treat in detail of the 

 Ostracoda and Copêpoda, of which several interesting forms 

 have been reared in my aquaria. 



The plates accompanyng this paper have been prepared 

 with the outmost care by the aid of the autographic me- 



