EXPLANATION OF PLATE

a, anus; ad, anterior adductor muscle; bc, branchial chamber; bgl, byssus gland; c, chitin; e, eye spot; f, foot; g, gills; h, heel of foot; i, intestine; 1, liver; lp, lower lip; m, mantle; mo, mouth; oe, œsophagus; og, supra-csophageal ganglion; ot, otocyst; pd, posterior adductor muscle; pg, pedal ganglion; 8, larval shell (prodissoconch); s', spat shell (dissoconch, nepionic); sbc, suprabranchial chamber; st, stomach; v, velum.

FIGS. 1, 2, 3, 4, 5. Development of Canadian oyster from bivalve veliger to young spat. From the right side, drawn under the same conditions throughout, Leitz microscope, oc. 3, obj. 2 (revolver). Zeiss Zeichenapparat. Drawing desk flush with stage and slanting upwards at proper angle to prevent distortion.

Measured under Leitz oc. 5 and obj. 4 with a Leitz oc. microm. valued by a Leitz stage microm.

Oyster larva, young straight hinge stage. .089 mm. high, .103 mm. long.

Oyster larva, early umbo stage. .138 x.144 mm.

Oyster larva, full grown. .31 x.34 mm. Fig. 3' from the left side. Oyster spat, with the valves of the larval shell (prodissoconch) plainly retained. .51 x.55 mm.

66

5.

Oyster spat.

66 6.

.876 x 1.030 mm. The larval shell is .369 x.384 mm.

Same as Fig. 1 drawn under oc. 3, obj. 4, for comparison with Figs. 7, 8, 9.

FIGS. 7, 8, 9. Same as Fig. 3 drawn under oc. 3, obj. 4.

FIG. 7.

46

8.

46 9.

Oyster larva from the left, with several organs sketched in.

Same from the right, with dorsal hinge-line tilted towards the observer. Same from the left, with ventral gaping margin tilted towards the observer.

FIGS. 10, 11, 12. Free-hand drawings of full-grown living oyster larvæ, not so highly magnified as Figs. 7, 8, 9.

FIG. 10. Oyster larva, full-grown, from the left, velum protruded and partly expanded.

44 11.

12.

Same from behind, attached to the slide by its fully expanded velum, large left valve.

Same from the ventral surface, velum partly protruded.

FIGS. 13, 14, 15, 16, 17, 18, 19, 20, 21. Sections of full-grown larvæ, drawn

FIG. 13.

free-hand.

Section median sagittal (nearly), from the right. The foot being turned sideways was not split medially.

had never seen an oyster larva or a young spat, but I had followed the main stages in the life history of the mussel.

Beginning my plankton-collecting at the end of the first week in July, it soon became apparent that there were many species of bivalve larvæ present in the water, and in order to refer these with some precision to the proper adults it would be necessary to carry on at the same time a faunistic study of the Mollusca of Richmond Bay. The commonest of these relative to my purpose were found to be species of Mytilus, Mya, Venus, Clidiophora, Ostrea, Anomia, Mactra, Modiola, Pecten, Saxicava, Macoma, Ensis, Yoldia, etc., and to find larvæ corresponding to all of them was beyond my ability. Nevertheless, several larval forms gradually became familiar and I referred them provisionally to certain adults. On the twenty-fifth of July what I took for oyster larvæ (Plate, Fig. 3) first decidedly claimed my attention and as time went on I became more and more convinced of the correctness of my surmises. But belief is not proof, so I set to work experiments with a view to entrap oyster larvæ on glass plates at a time when presumably the larvæ become too heavy to swim with ease, settle towards the bottom, creep about and select some clean, solid surface upon which they fix themselves and transform into the youngest oyster spat. This was successfully accomplished on the sixteenth of August when I obtained a minute oyster spat (Fig. 5) still preserving most evident characteristics of the larva, but with the addition of a rim of spat-shell, and later I found many minute spatoysters on various natural objects such as shells and stones.

The plankton was collected in conical nets, made of finemeshed silk bolting cloth, attached at the broad end by a rim of linen to an iron ring one foot in diameter, to which were tied, at equal distances, three pieces of codline, the other ends being brought together and secured to a towing line. The small end of the net was also

furnished with a linen rim in which was tied the neck of a wide-mouthed bottle. To the towing line, in front of the net, was fastened a sinker and the whole was dragged through the sea-water, behind the little steamer Ostrea, under reduced speed, for about a mile, when the net was hauled up, the contained water carefully drained through one side, after which it was dipped several times right side up into the sea and raised so as to wash all the plankton material down into the bottle. The bottle could then be removed and corked, the net washed by throwing it overboard again open, and other bottles used for different places or different depths on the same

excursion.

In such a manner may be procured a wealth of plankton material, but slight modifications in mode of operation may be employed, depending upon the nature and object of one's research. The older bivalve larvæ are compact, heavy, well protected, so that they will stand comparatively rough usage. By the time one reaches the laboratory the great mass of the copepods may be dead and sunk towards the bottom of the bottle, but underneath this mass one can see the darker, granular, more sand-like bivalves. These may be withdrawn by a glass tube and emptied into a watch glass, the more superficial, lighter things being again removed by a pipette. In this way bivalve larvæ may be obtained, sometimes by thousands, and almost entirely free from admixture with other animals, while among them, if collected at the proper time and place, will occur oyster larvæ.

At Malpeque the full-grown, free-swimming, pelagic, or more or less abyssal, or creeping larva of the oyster (Figs. 3, 7, 8, 9) possesses a characteristic brownish-red color-suggestive of the soil of its native island shores-a shade which enables it to be immediately distinguishable from every other bivalve larva with which it is associated. The shell (prodissoconch) is asymmetrical, inequivalve and inequipartite, the left valve being larger, more convex and with a large umbo, the right one smaller, flatter

and with a moderate umbo, while the umbos have a postero-dorsal position, projecting backwards and upwards and making the shell broader, deeper, squarer behind and tapering but rounded in front. The largest measure about .358 by .365 mm. in height and length, but, owing to the different convexities of the valves, the greater breadth above and behind, and the different degrees to which it may be tilted in this way or that, the same larva may vary much in apparent size and shape according as to how it is presented to the observer. The following are measurements of half a dozen larvæ at different ages selected from a large number of records: .131 x .138 mm., .138 x .144 mm., .207 x .241 mm., .241 x .276 mm., .296 x .345 mm., .345 x .372 mm. The larval shell of the young spat (Fig. 5) measures .369 x .384 mm. and may be taken to represent the maximum size.

When mounted on a slide the larvæ are accustomed to remain quiescent, and from their deep coloration are difficult to examine, but sometimes a more transparent one permits certain organs to be traced. When freshly collected and examined in a watch-glass of pure, cool water from their native habitat, many of them exhibit the greatest activity, swimming hither and thither or circling round and round by means of the velum (Figs. 9, 10, 11, 12), a swimming organ which they protrude between the antero-ventral margins of the shell-valves and expand in a manner resembling the opening of an umbrella. The margin of this is densely covered with large cilia, the violent flapping of which propels the animal forwards with the heavy body and shell suspended beneath the velum. Jarring the watch-glass will cause the animal to immediately withdraw its velum (Figs. 7, 8), at the same time snapping the valves of its shell together and dropping towards the bottom. Such observations illustrate the ordinary mode of locomotion and the response to violent movements in the sea, for during heavy gales a plankton net will take few or no larvæ near the surface. An organ of immense interest to zoologists and of vast