INTRODUCTION 5 



and also that some were more pointed at one end than others, or were 

 shorter and deeper. Here then, were differences of size, colour and shape, 

 but I did not know whether they meant numerous different species or dif- 

 ferent ages of the same or a few species. I then started with one of the 

 largest and commonest, and tried to search out a series descending in size 

 but retaining the same shape, and I found that the smaller (i.e. the younger) 

 were paler and more transparent than the larger (or older) of the same 

 series, but that there were large ones of a different shape that were paler 

 than small ones of this series. A second series was begun for them, and so 

 on, until I had in mind notions of several different species without knowing 

 what species. I reasoned that the mussel (Mytilus), the clam (My a), the 

 quahaug (Venus), and the oyster (Ostrea) furnished the commonest shells 

 collected, and consequently they were likely to supply the commonest 

 larvae also. Of these I knew the largest larva of the mussel — of that there 

 was no doubt whatever, for the smallest mussels attached by their byssus 

 to sea-weeds, rocks, weirs, wharves, and such objects, showed narrow, 

 concentric blue rims round a somewhat horn-coloured umbonal area of 

 the same size and shape as the shell of the largest free-swimming larva of 

 the plankton, and by extending one of my series upwards until it con- 

 nected with this and downwards as far as it could be traced, I not only 

 learned to recognize the mussel at every size intervening between these 

 limits, but removed all these stages of this most abundant larva from the 

 field of question, thus narrowing the problem closer around the oyster. 

 I tried in the same way to eliminate other species, but for them the stages 

 immediately following the free-swimming larvse are not so easily found as 

 in the mussel. All the while I was extending my series and forming 

 theories, which were little better than guesses, as to what they were and 

 which was the oyster. 



A peculiar, opaque, reddish-brown coloured larva, that did not always 

 look the same in shape, sometimes appearing so aberrant as to resemble 

 certain broad-mouthed, low-conical, univalve shells, had been singled out 

 as worthy of special attention. But to prove whether this was the larva 

 of the oyster or not I must obtain the youngest fixed stages (spat) follow- 

 ing the largest free-swimming plankton stages. I judged that such stages 

 for the oyster would probably be attached to rocks or shells, whereas the 

 corresponding stages for the clam and quahaug would be free-creeping 

 and burrowing, like their parents. 



I examined the surfaces of shells, stones, rocks, stakes, wharves, the 

 superficial ooze, mud, sand, gravel, rock-weed, eel-grass, and everything 

 in fact I could think of in various parts of the bay. But what small bi- 

 valves I could find were not small enough for the purpose, because, as a 

 shell grows bigger, the larval shell retained in the region of its umbo be- 

 comes weathered, corroded, overgrown, thickened or warped, its valves 

 pressed open, rent apart or otherwise so far distorted in appearance and 



