56 COMMISSION OF CONSERVATION 



grown oysters that carry periwinkle shells at their umljos, it would seem 

 that these latter are a common base of fixation, but the dark colour of the 

 winkle and the colonies of Ralfsia verrucosa it frequently bears render it 

 difficult to find very young spat on its surface. On the other shells, after 

 being shown specimens, the men on the steamer "Ostrea" could also find 

 quite young spat. 



Dimensions of Newly Fixed Spat. — The spat caught on glass varied 

 of course in size — the first measured .87 x 1 .03 mm. in height and length, 

 the second 1 . 58 x 1 . 20, the third . 51 x . 55, the fourth . 86 x . 95. Similarly 

 the first found on an oyster shell measured 2.4x2.3, while those subse- 

 quently procured varied from less than 1 mm. to 6 mm. in height. Of 

 these latter a verj^ large number was collected, so that I could easily arrange 

 series passing by small gradations in size towards the larger spat of the 

 fishermen. But of the smallest (just attached) spat I had few specimens. 



Five years later, however, in 1909, I again had the opportunity 

 of pursuing the subject, and I procured an abundance of the very 

 youngest spat — many of them in fact slighth^ smaller than some of the 

 largest free-swimming larva^ caught in the plankton net, which shows 

 either that there is a certain individuality or that there is some ability to 

 accommodate themselves to circumstances. 



The largest larva I have a record of, measured 56 units (= .386 mm.) 

 in length. The smallest spat I have found measured 53 units (= .365 mm.) 

 in length, and I can state that it was normal in both fixation and structure, 

 since I have a complete series of transverse sections of it. It would take 

 nearly 70 of these placed end to end to reach over a length of 1 inch, and 

 5000 side by side to cover a surface of 1 square inch. There was a visible 

 narrow rim of spat-shell added to the edge of the larval shell, and the fix- 

 ation could have been at most only a few hours old. 



A common size for the fixation of the larva is 55 units (= .379 mm.), 

 as shown by the number of spats of this size obtained, as well as by the 

 measurements of the prodissoconchs in the umbonal regions of small spats. 

 Some of this size had no visible addition of spat shell. 



Sizes of young spats I have recorded are 53, 55, 58, 60, 62, 65, 67, 68, 

 70, 72, 76, 80, 82, 83, and upvv^ards, but I have many specimens which, 

 if selected out and measured, would very likely fill in the intermediate 

 stages. 



References to the Spat. — Sprat (1669, p. 307): "In the month of May the Oysters 

 cast their spawn (which the Dredgers call their spat) ; it is like to a drop of Candle, 

 and about the bigness of a half-penny. The spat cleaves to stones, old Oyster-shells, 

 pieces of Wood, and such like things, at the bottom of the sea, which they call 

 'Cultch.' 'Tis probably conjectured, that the spat in 24 hours begins to have a 

 shell The Oj'sters when the tide comes in, lie yvith hollow shell down- 

 wards, and when it goes out they turn on the other side; they remove not from their 

 place unless in cold weather, to cover themselves in the Ouse. The reason of the 

 scarcity of Oysters, and consequently of their dearness, is because they are of late 



years bought up by the Dutch The Oysters are sick after they have spat; 



but in June and July they begin to mend, and in August they are perfectly well. The 



