34 COMMISSION OF CONSERVATION 



Younger stages are also obtained in this way, but, on account of 

 their lighter weight, do not so readily make their way to the bottom. 

 Even pre-conchiferous trochophores and segmenting stages may be pro- 

 cured, but in collecting them it becomes necessary to exercise greater 

 care. The high speed and rough usage with which the shell-bearing 

 larvae may be safely collected might prove completely destructive to the 

 delicate shell-less stages. The method, however, is of no special merit, 

 except as a proof of occurrence, in the study of these young stages, for 

 they can be obtained with greater ease and in vaster numbers by arti- 

 ficial fertilization, and besides the latter has the additional advantage 

 that the parents are already known. 



Identification of Oyster and Other Bivalve Larvae. — Identification for 

 the first time of oyster larvse in plankton collections was difficult, but, 

 when once determined, at least the older stages could be recognized al- 

 most at sight in succeeding catches. Some account is given in the intro- 

 duction of how this was first accomplished. It scarcely needs stating 

 that the plankton net collects whatever comes in its way, and that there- 

 fore oyster larvae will form but a small part of the total catch. The 

 method I have adopted separates the bivalve larvte from a great mass of 

 other things, but, even with this done, the bivalve larvse of themselves 

 represent many species, and it becomes a question how to distinguish the 

 oyster larvge from the rest. 



Larvae do not look like adults. Those of the same species at distant 

 intervals of time may appear as unlike one another as different species. 

 It is difficult to find a point of departure in distinguishing them. One 

 may begin with the size, the shape or the colour, but after a time find that 

 he cannot trust to impressions and memory. He may recognize dif- 

 ferences, and gradually arrive at the conviction that certain of these are 

 constant, but later on find out that there are intermediate conditions. 

 He may notice that certain characters are associated, and have to go over 

 his collections again to decide upon combinations, such as colour and 

 shape, length and depth, and discover that he cannot safely trust to the 

 eye in judging proportions. All of this takes time, but is a clear gain in 

 experience and knowledge, and begins to determine a definite point of 

 view and a consequent mode of procedure. Having reached this point, I 

 started to make new sketches with such arrangement of oculars, objectives, 

 and drawing apparatus that I could obtain accurate outlines of all stages 

 from the smallest to the largest larvae under exactly the same conditions, 

 and at the same time make use of a definite and unvarying method of 

 measuring b}^ means of ocular and stage micrometers. Loose sheets of 

 paper were used so as to permit of easy re-arrangement; on the right side 

 was placed a table of lengths proceeding by one of the smallest units of the 

 ocular micrometer at a time, while on the left was kept a space for the 

 sketch. Then larvae were sought out corresponding to these lengths and 



