128 THE SCALLOP FISHERY 



(b) Microscopical Examination. This method was used to more 

 or less extent with (a), and was only of additional value in following 

 the development of the immature eggs previous to the spawning season, 

 showing at what period other investigations should be started. The 

 eggs and sperm were removed from the ovary, placed on a slide and 

 their size and appearance recorded, the sperm being classed as (1) 

 active or (2) inactive. 



(c) The Plankton Net. A small net of silk bolting cloth No. 11, 

 with a diameter of 12 inches, and slightly tapering for 24 inches to 

 a rounded bottom, was used for this work. By towing the net through 

 the water the veliger larva?, which are abundant during spawning 

 season in the water, could be captured. This is an important method 

 of recording the spawning, as the presence of scallop veligers from 

 two to four days old is proof that the spawning season is under way. 

 By making daily towings under the same conditions and for a definite 

 distance, if was possible to count the number of larva? in the water 

 each day, and thus determine the conditions influencing the spawning- 

 season. Although this method has been of greater value in the work 

 on the other shellfish, as the same method is applicable to lamelli- 

 branchs in general, a description is here given. 



The plankton net, as shown in Fig. 72, is attached in the form of 

 a bag to a copper ring, to which the tow line is fastened in the same 

 manner as a kite string. The outfit is trailed from the stern of a dory 

 or rowboat for a definite distance at a slow, uniform rate, so that no 

 outward current will sweep away the larva? from the mouth of the 

 net, which acts as a sieve to collect all microscopic organisms too large 

 to pass through the meshes. 



When the proper distance is covered, the net is taken from the water 

 and the contents washed into a small pail containing from 4 to 5 inches 

 of clear sea water. The lamellibranch and gasteropod larva? are now 

 separated from the rest of the tow contents by giving the water a swift 

 circular movement around the edge of the pail with a small stick. The 

 action of the water forces the larva? to settle to the bottom at the 

 center of the pail, where they can be readily transferred by a pipette 

 to a watch glass for study. 



A convenient means of analyzing the towing similar to the Sedgwick- 

 Rafter method of a diatom counting was devised. The larval contents 

 of the towing was spread evenly throughout a cell 50 by 20 by 1 mil- 

 limeters, covering an area of 1,000 square millimeters, or a cubic vol- 

 ume of 1 cubic centimeter, and ten counts (Moo of the total area), 

 each covering an area of 1 square millimeter as measured with a square 

 ocular micrometer, were made from different parts of the cell to get 

 a representative average. The approximate number of larva? for each 

 species of shellfish was obtained by multiplying the sum of these counts 

 by 100. 



