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falls between July eleventh and August sixteenth. king it that the youngest larvæ I have described are little older than those of similar shape and structure described by Brooks, and allowing a possible retardation on account of the climate, we should conclude that the eggs were deposited pretty close to the first of July. That spawning does not take place much before this I judge from the fact that in 1905, while I was at Malpeque preparing the station for removal, I took plankton at intervals between June seventh and twenty-sixth and this shows no oysters and but few mussels and clams.

In the microscopic examination of the genital organs for the purpose of determining the time of sexual maturity, unless one examines a very great number taken from many different localities, he may light upon an abnormal number of individuals that are immature or that have already spawned and so form a wrong conception as to the period of maximum spawning. Combination of both methods should give the best possible results.

I have purposely attempted to disregard the statements of others in order to be entirely unbiased as to my results, and from the facts of my own observations I am disposed to think that the period of maximum spawning falls in July, but that a few may spawn earlier and a greater number may straggle in later.

It is a matter of regret to me that it did not fall to my lot to begin the study of oyster larvæ during the first summer at Malpeque, for then I could have used the second summer to verify, fill in details, and follow out suggestions. I have looked forward ever since for an opportunity to do so, and this is my chief excuse for the delay in publishing these results.

Chief points of importance resulting from the foregoing work are:

1. Larval oysters are present suspended in the water of Richmond Bay, Prince Edward Island, in July and August.

2. They may be taken in a plankton net at the surface and at various depths.

3. All stages from the freshly fertilized egg to the full-grown larva must be there.

4. The free-swimming period is, perhaps, considerable, close on a month.

5. They feed and grow, while in the free-swimming state, through a straight-hinge to an umbo-stage.

6. Normal fixation takes place when the larval shell is about .38 m. in length, and then the spat period begins. 7. A metamorphosis occurs through loss of larval organs as velum, foot, eye-spot, otocysts, etc., and a development of new organs as spat-shell, additional gills, palps, etc., is begun.

8. The larval shell is asymmetrical, as is also to some extent the contained body.

9. A foot, homologous with that of mollusks in general, is present in the older larvæ.

10. The otocysts contain otoconia.

11. Pedal ganglia are present.

12. A byssus-gland is present. 13. Gills are present.

14. Eye-spots are present.

15. A rigid system of measurements has been used, and a comparison of actual sizes at different periods of growth introduced.

16. Numerous niceties of structure, shape, color, activity, time, place, etc., are noted.

17. The spawning period has been limited.

18. Attention is directed to the importance of these theses and observations as bearing upon problems and methods of oyster culture.

LITERATURE

European works referring to the development of the oyster larva are those of:

1. 1854.

Lacaze-Duthiers. Mém. sur le dévelop. des Acéphales Lamellibr.
Comptes Rendus heb. des Séances de l'Acad. des Sciences,
Paris, XXXIX, pp. 102–106. Nouvelles observ. sur le
dévelop. des huîtres. Same vol., pp. 1197-1200.

2. 1882 ('83). Horst. A Contrib. to our Knowl. of the Develop. of the

3.

4.

Oyster (Ostrea edulis L.). Bull. U. S. Fish Com., II, pp. 159-167, 12 figs.

1884 ('86). Horst. The Develop. of the Oyster (Ostrea edulis L.). Rep. U. S. Fish Com., pp. 891-910, Pls. I and II.

1883. Huxley. Oysters and the Oyster Question. Eng. Illus. Mag., Oct. and Nov., pp. 47-55, 112–121.

American work must be considered to have originated with Brooks, whose discoveries inspired investigators at home and abroad and pointed the way to possibilities and methods of culture that were ably carried forward by Ryder, Winslow, Rice and others. Of the many papers, reprints, summaries, etc., published by Brooks I mention but one:

5. 1880. Brooks. Development of the American Oyster. Rep. of the Com. of Fish. of Maryland, pp. 1-18, 10 pls.

6.

7.

8.

1882 ('83). Ryder. On the Mode of Fixation of the Fry of the Oyster. Bull. U. S. Fish Com., II, pp. 383-387, 9 figs. 1882-83 ('84). Ryder. A Sketch of the Life-history of the Oyster. 4th Ann. Rep. of the U. S. Geol. Surv., pp. 317-333, pls. 73-82.

1882 ('84). Ryder. The Metamorphosis and Post-larval Stages of Development of the Oyster. Rep. U. S. Fish Com., X, pp. 779-791.

9. 1884. Ryder. A Contrib. to the Life-history of the Oyster. Fisheries and Fishery Industries of the U. S., Sec. I, pp. 711–758. 10. 1882 ('84). Winslow. Rep. of Exper. in the Artif. Prop. of Oysters. Rep. U. S. Fish Com., X, pp. 741-762.

11. 1889. Jackson. The Develop. of the Oyster with Remarks on Allied Genera. Proc. Bos. Soc. Nat. Hist., XXIII, pp. 531-556, 4 pls.

12.

1890.

Jackson. Phylogeny of the Pelecypoda. Mem. Bos. Soc. Nat.
Hist., IV, pp. 277-400.

Canada has done little towards a scientific study of oyster development. Three rather unpretentious articles are known to me.

13.

1895 ('96). Prince. Peculiarities in the Breeding of Oysters. Special Reports, Ottawa, pp. 10-13.

14. 1904. McBride. The Canadian Oyster. The Canadian Record of Science, Montreal, IX, July, pp. 145-156, Figs. 1-4.

15. 1905. Stafford. On the Larva and Spat of the Canadian Oyster. THE AMERICAN NATURALIST, Boston, pp. 41-44. (Preliminary to this paper.)

BRIEF NOTES AND CRITICISMS

Brooks (No. 5, p. 25, of the preceding list) says: "All my attempts to get later stages than these failed, etc." He refers to his Figs. 44 and 45 which were perhaps a little younger than my Fig. 1 and were six days old. I never could understand the claim that they might develop to this stage in twenty-four hours.

Horst (2, 165; 3, 904) was unable to get stages older than his Fig. 12, a straight-hinge shell of .16 mm., which according to Ryder (7, 791) would be equivalent to an American larva of half this length, i. e., little younger than my Fig. 1. He adds: "I have also been disappointed in my attempts

to procure oysters in these phases of development by means of catching larvæ floating about in the sea.”

Ryder's papers (6, 383; 7, 328–9; 9, 727) are not easy to correlate on account of discrepancies in measurements or magnifications, age and occurrence, a misuse of the term embryo, etc. Making allowance for these, and combining his statements in the probable order in which they were written, I conclude that Ryder had seen two stages approximately of the same age as two I have figured. His Fig. 1, magnified 183 times measures 14.5 mm., which would give the larva an actual length of .08 mm. His Fig. 3, mag. nified 96 times, measures 29 mm. and similarly gives the young spat an actual length of .3 mm. But his were both fixed, and in fact it was this property which in Ryder's methods afforded the chance of their being observed. He appears to have believed that the larger one (C. 0 in.) marked the proper stage of fixation but that under "favorable circumstances'' larvæ of the size of the smaller might become fixed and then grow to the size, shape and structure of the larger, at which time they first become spat. Considering that he obtained the small ones but once, that they were attached in no regular position, and that the one figured was on its right side instead of on its left, it seems more probable that the fixation was of a transitory nature (as regarded by Jackson) or that it was abnormal, due to unfavorable artificial conditions and that the normal process is for the larva to remain free until it reaches the size of the prodissoconch in the umbo of the young spat shell. Ryder's view of the duration of the free-swimming period as limited to twenty-four hours comes nearer to a possibility if we remember that he doubtless had in mind this case of abnormally early fixation. A similar statement might be made with regard to the sentence 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.'' These also agree very well with the larvæ raised by Brooks and by Horst. He never saw larvæ between these two stages in size. This represents a period during which the larvæ had to grow to nearly four times their former diameter and undergo a very great increase of organization. If the smaller stage can be raised as in Brooks's larvæ in six days the larger might require four times six days additional, making a month for the complete larval development. This time according to Brooks, Ryder and others might be reduced by very warm weather. It is just possible that too high a temperature of small isolated quantities of water may be one of the adverse conditions which have prevented larvæ from being raised beyond this stage. In nature they not only have a broader source of food supply but they can also sink into cooler water.

Winslow (10, 757) thought that the oyster larva is predisposed to fix itself very soon after segmentation and when the shell is developed to a slight extent the larvæ remain quiet in one place at the bottom. I can believe that they do not by their own efforts travel very far from the place of their origin for their locomotion is largely a circling or to-and-fro movement, but while suspended in the water they may be transported by tidal currents.

Jackson (12, 300) wrote: "Between the stage Fig. 25 and our next stage, Pl. XXIV, Figs. 1, 2, there is a blank in the knowledge of the

In

development of the oyster. It has not been described in the European species, and all attempts to obtain it in our own species have failed. artificial confinement the oyster dies at this stage." His Fig. 25 is Huxley's cut of a straight-hinge larva. Figs. 1, 2 of Pl. XXIV are Jackson's own youngest stage of the spat, obtained August 4, 1888, on glass put in a drain-pipe trap on a sand-bar in Buzzard's Bay. It was firmly attached by the ventral margin of the left valve and as it had been attached less than twenty-four hours the anatomy and shell must have been developed while it was a free larva. The figures measured 37 mm. and as they are magnified 120 diameters the actual length of the recently free larva, now a fixed spat, was nearly .31 mm.

Prince (13, 13) makes the statement: "I captured many small embryooysters several miles from any known oyster areas,' "' but as no measurements or drawings accompany the paper one can not judge of their size

or age.

....

McBride (14, 151, 153) says: "Judging from the size of free-swimming larvæ caught by the tow-net ... During the latter part of the month (August) the waters were swarming with larvæ which, from their exact agreement in shape and appearance with the larvæ of the European oyster, were doubtless the later stages of the free-swimming young of the Malpeque oyster. . . . The later larvæ which were captured by the tow-net are characterized by possessing a straight hinge to the shell. Fig. 4. Late Larva of the Oyster captured by the Surface-net." The so-called late larvæ are in the light of my researches in reality somewhat early larvæ. Late larvæ would be more appropriately applied to the umbo-stage which doubtless was in the water at the time referred to, but was at that time unknown as a plankton organism. The statement that the waters during the latter part of August were swarming with young straight-hinge oysterlarvæ does not correspond with what I found at the same place the succeeding year. Upon examining Fig. 4 I find that it is not an oyster larva. The measurements are 83, 70, 51 mm. which if divided through by 5.53 will give 15, 12.6, 9.2 mm. as the length, height and hinge-line. Referring this to the table of comparison of a mussel, a clam and an oyster at this period, on a former page, it becomes evident that it could have been no other than the larva of the clam.

The shell of the larva was held to be perfectly symmetrical by Ryder (6, 384; 7, 329; 8, 787; 9, 727), but Jackson (11, 541; 12, 312) observed in his youngest spat that the lower left valve was larger and deeper than the upper right one.

A foot has been mentioned by Lacaze-Duthiers, Horst, Brooks and Jackson. Lacaze-Duthiers (1, 105) said: "En avant de l'anus un appendice pen saillant simule un rudiment de pied." Horst (2, 162) stated that "A foot-like prominence is developed, whereby the animal assumes some likeness to a young gastropod.'- Brooks (5, 53) wrote: "Near the center of the ventral surface-the top of Fig. 32-there is a well-marked and constant protuberance of the body wall, which occupies the region which, in most molluscan embryos, gives rise to the foot, and which may perhaps be regarded as a rudiment of that organ." In the same paragraph and referring to the same figure he mentions "the primitive digestive cavity" and on page 68 "the primitive digestive tract opens by a wide blastopore.''

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