RESUME OF THE STAGES OF DEVELOPMENT 83 



characteristic shape and were easily counted. Prepared in the way- 

 mentioned there would be about 25 oyster larvae under a f inch cover-slip 

 and about seven times this number of other bivalve larvae. The numbers 

 kept up for a few days into August and then began to fall off, at first 

 slowly, but from the 20th rapidly, and after the first of September scarcely 

 an oyster larva could be found. In 1909 the season was cold and 

 backward. Small larvse first occurred on July 22nd (at Cocagne). The 

 numbers increased beyond previous observations. At Bay du Vin on 

 August 5th, I counted 83 oyster larvse under a f inch cover-slip. At Mal- 

 peque on August 26th I counted 35 in the field of view (Oc. 3, obj. 2). 

 On August 30th I counted 150 under one cover-slip. I have preserved 

 plankton for September 3rd that shows an average of a dozen oyster larvae 

 in each field of view. I did not wait to see the decline of the oyster season 

 that year, but I calculated from the appearance of the first larvne that the 

 season was about three weeks late. 



The rate of growth of oyster larvae cannot be determined by keeping 

 them confined in small glass vessels where they may be easily found and 

 observed. The temperature and aeration conditions are unnatural and 

 the food supply is shut off. Close observation of plankton-catches at the 

 beginning of the season will show that the largest larvae of one day's col- 

 lection will be larger than the largest of a previous day. At Malpeque, 

 July 11, 1904, the oyster larvae taken could be arranged according to 

 length from 12 to 20 (-083 mm. to -138 mm.) On the 14th lengths'^of 

 21 to 26 (-145 mm. to -179 mm.) could be added. On the 27th from 30 

 to 46 (-207 mm. to -317 mm.). On August 3rd, from 47 to 52 (-324 mm. 

 to -358 mm.) This would seem to show that it requires at least three 

 weeks to grow from the smallest shell-covered stages to the full-grown 

 larvae. In 1909 the first oyster larvae of July 22nd at Cocagne measured 

 10 to 14, those of July 26th, 11 to 27, those of August 2nd, 29 to 38. On 

 August 4th, at Bay du Vin, I first observed full-grown larvae of 54 units 

 (about 1/66 inch) length among numerous others ranging from 29 to 50. 

 From this it would appear that the larvae grew from the smallest to the 

 largest shelled stages in about two weeks. There is a possibility that Bay 

 du Vin larvae were slightly in advance of those of Cocagne. I had taken 

 plankton there on July 13th, nine days before the first larvae occurred 

 at Cocagne, and there were no oyster larvae at that time, so that any ad- 

 vance was less than nine days. Lateness of spawning, a continuous and 

 very warm spell after the long cold spring, abundant food or other causes, 

 may have hastened development. But, if so, these causes did not shorten 

 the season, for there were swarms of larvae in the water at Malpeque on 

 September 3rd. I prefer to think that the season of 1909 was not so near 

 the average as that of 1904, and that three weeks is nearer than two weeks 

 for the average normal growth of the larvae through the shelled stages. 



