METHODS OF COLLECTING AND PHOTOGRAPHING 67 



to enter the box through the cylinder of netting and are thereby 

 saved from drowning which would ensue if they could not reach 

 the air. They may be removed through the lid at the convenience 

 of the collector. 



II. Invertebrates 



Invertebrates are to be collected in three situations: in the 

 aquatic vegetation bordering the shore, in the open water, beyond 

 this vegetation-zone, and on the bottom, so that the apparatus 

 suitable to each of these situations may be separately considered. 



It is convenient to consider first those methods designed for 

 quahtative work, for finding out what organisms are present, and 

 second those methods by which the number or quantity of organ- 

 isms present in a unit volume of water or under a unit area of sur- 

 face may be determined. 



A. Collecting in Littoral Vegetation 



1. By dip nets. The dip net (Fig. 10) is here of greatest use. It 

 consists of a conical netted bag about one foot in diameter and 

 eighteen inches deep attached to a 



stout ring of brass or iron, firmly 



fixed to a stiff, wooden handle seven 



or eight feet long. The lower third of 



the net may often be advantageously 



lined with thin, cotton cloth to retain 



smaller organisms. A form of this net 



adapted to scraping flat surfaces, such ^^^ 



as logs, flat stones, banks, etc., is also ^ 



, /_,. . ^ - . . , Fig. 10. Two forms of dip net. For de- 



ShOWn (Fig. 10). It has a Semi-CirCUlar scription see text. (From photographs 



rim and a shallow bag of canvas with 



a bottom of No. 6 or 8 bolting cloth. The handles used on dip 

 nets are rake handles. The iron rings may be made by any 

 blacksmith. The bags are sold as minnow dip nets by dealers 

 in fishing tackle or by mail-order houses. 



2. By collecting larger aquatic plants. With such nets many 

 forms visible to the naked eye may be collected directly, or the 



