754 



FRESH-WATER BIOLOGY 



For preservative fluid either 5 per cent formol or 75 per cent 

 alcohol can be used. Alcohol preserves the animals in a little 

 better shape, as formol is apt to make them brittle. Dissection is 

 best performed in a drop of glycerin on a slide. The transfer 

 from alcohol to glycerin should be made gradually, through mix- 

 tures of alcohol and glycerin. The larger forms of Copepoda can 

 be dissected under lenses magnifying from five to fifteen diameters; 

 but, for the smaller individuals, powers as high as one hundred 

 diameters must be used, and the work is very tedious. The 

 dissected material is best mounted for examination in Farrant's 

 solution, and the cover must be ringed with a good cement, — 

 Brunswick black, for instance. 



If one wishes to make a serious study of the animals, the struc- 

 tures to be separated and studied are the following: the antennae, 

 male and female; the abdomens, male and female; the fifth feet, 

 male and female. In addition, the general form of the cephalo- 

 thorax must be noted, and, in some cases, the structure of the other 

 appendages of the cephalothorax. The Copepoda are so widely 

 distributed and their forms are so characteristic of biological con- 

 ditions that it is very desirable that every student of water forms, 

 even if his work is only of an amateur character, should be able 

 to make a separation into genera, and, in most cases, make at least 

 a provisional specific determination. Fortunately, the generic dis- 

 tinctions of our American forms are very easily made, and it is not 

 difficult to recognize some of the more common species. 



Especially confusing to the beginner is the large number of im- 

 mature forms. Many of the larval stages of the more highly de- 

 veloped species resemble closely the mature forms of the simpler 

 species, so that the tyro is apt to think that he has a large number 

 of species, when he may have only several stages of one. It is 

 safest for the beginner to make no attempt at identification except 

 in the cases of evidently mature forms, such as egg-bearing females. 



The Copepoda are an order of Crustacea, belonging to the sub- 

 class Entomostraca. Tliis subclass cannot be easily defined, but 

 it is sufficient for our purposes to say that they are the most simply 

 organized Crustacea. The Copepoda may be defined as those En- 

 tomostraca which do not have a shell-Hke covering of the body 



