LARVAL OR SWIMMING STAGES 39 



The temperature was 72° to 80° F. The fry (young straight-hinge larvse) remained of 

 about the size of the eggs put into the hatching apparatus, containing filtered sea-water, 

 and did not grow any during the three days under observation. He did not know how 

 they were attached, but they lay upon the side or in other positions, with the border of 

 the mantle projecting over the edge of the shell, and could not be washed away by a 

 stream of water. They could only partially retract velum and mantle. The shell was 

 perfectly symmetrical; hinge-line straight, without umbos. The adhesive organ was 

 probably the border of the mantle, although an unobservable temporary byssus may 

 have existed. Attempts to repeat the experiment failed. His Fig. 1 (" Young American 

 oyster two days old, magnified 183 times ") measures 14 • 5 mm. as length of shell, which, 

 when reduced to the actual length (14-5 -f- 1S3) gives -08 mm., somewhat larger than 

 an egg, but near enough to justify his statements. This agrees closely with the figures 

 42 and 44 by Brooks, and to one familiar with the subject can be recognized as of ap- 

 proximately the same stage, notwithstanding the absence of measurements from Brooks' 

 work. It is evident that the age (two days in one case, and twenty-four hours to six 

 days in the other) can not be depended on in making comparisons, and the larvse ob- 

 served by Ryder would have measured the same at the end of five or any other number 

 of days as at the end of two, since they did not grow. Hence the necessity of taking 

 account of measurements and organization. 



For a reference to Ryder's 1882-3 paper, see p. 30. 



At the same place, in August of the same year, Ryder obtained young stages of 

 spat, of which the youngest was his Fig. 3, magnified 96 times. The figure measures 

 29 mm., which gives an actual length of -3 mm. (sufficiently correct). 



For a quotation from his 18S4 paper, see p. 30. 



Ryder, we conclude, had two stages, represented by his Fig. 1 and Fig. 3, both at- 

 tached, and in fact it was due to their attachment that, with his methods, he was able 

 to find them. The younger agreed in size, age and appearance with straight-hinge 

 larva?, raised by culture; in reality these were so obtained. The older, on the other 

 hand, agreed in size and appearance with young spat that had " only just begun to de- 

 velop the spat shell." He could not correlate the two stages of attachment except on 

 the supposition that " under favourable circumstances attachment of the fry probably 

 takes place within twenty-four hours after fertilization The young, how- 

 ever, after attachment continue to grow as larvae When the valves of the 



fry have acquired umbos the development of the spat shell begins." We know now 

 that Fig. 1 represented an abnormal attachment, most likely due to unfavourable (rather 

 than 'favou able') artificial conditions. Ryder was confined to a single observation 

 and he did not know that there were in the open waters of the sea, perhaps at the very 

 moment, incalculable millions of free-swimming umbo-stages that would bridge the 

 interval of development between his two fixed stages. He was himself suspicious that 

 the youngest were not normally attached for he remarks: "Our conclusion was that 



these young embryos had voluntarily attached themselves this young 



fry had a disposition to lie upon the side Many were noticed in other posi- 

 tions, but I am inclined to believe that these were not normal." He should also have 

 noticed that the one he selected to draw was not normal, in that it was attached by its 

 right side instead of the left. Jackson (1S90, p. 300) referring to this very case of Ryder's 

 stated : " From my own observations I have every reason to believe that the fixation was 

 of a transitory nature." Attachment at this size is certainly exceptional and unnatural 

 and these were certainly not normally attached by the shell but likely clung by the pro- 

 truded edge of the velum or mantle. Yet the single observation of this abnormal case 

 influenced Ryder's views to the extent that we must continually keep it in mind in 

 trying to understand his statements. If this fixation were regular then the period of 

 free larval life preceding it must have been very brief, for fertilized eggs had been raised 

 to this stage and the period was approximately known. Hence the statement that, " The 

 difference in magnitude between the oldest artificially incubated fry seen by me and 

 that of the youngest fixed embryos which I collected is very small." The 

 "youngest fixed embryos" here referred to were those of Fig. 1, not Fig. 3. On 

 the other hand the period and the animal (whether free or fixed) between stages 1 and 

 3 were completely unknown: "The interval of development between that of our oldest 

 embryo with its diminutive Pisidium-like valves measuring about 1/500 inch in dia- 

 meter, and that of the embryo when its valves first begin to lose their embryonic form, 

 still remains unbridged." The first refers to Fig. 1, the last to Fig. 3. He was primarily 

 interested in determining the mode of fixation of the larva, but secondarily in estimating 

 the period of free existence by discovering the youngest stage of fixation. "The attach- 

 ment is effected very early, as I have met with it in spat a little over an eightieth of an 

 inch in diameter." This refers to Fig. 3, not Fig. 1. 



