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Volume 40, 1907
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Art. XXII.—Preliminary Note on some Stages in the Development of a Polychæte.

By H. B. Kirk, M.A., Professor of Biology in Victoria College, Wellington.

Plate XXIII.

In January, 1906, I obtained at Plimmerton some very interesting egg-masses of a Polychœte. The facilities for examination were not great; but I was able to keep the developing larvæ under observation for five days, although not without intermission. This year I failed in several attempts to obtain suitable material for further observation.

The masses were found cast up by the tide. In texture they are gelatinous, and in shape they somewhat resemble a barrel open at both ends. The length of the masses is from 20 mm. to 25 mm. The shape of any one of them leaves no doubt that it was produced by a worm provided with a cingulum.

In the gelatinous matter are imbedded eggs and larvæ, the latter in various stages of development, owing probably to fertilisation having been effected at different times. Each ovum has a thin membranous envelope, and, outside this, an envelope, possibly albuminous, more highly refractile than the surrounding gelatinous matter.

Embryos in the earlier stages of development were few. The earliest stage of which I could be certain was one in which eight megameres were surrounded by micromeres, apparently sixty-four in number, but of that number I could not be quite sure.

No trochosphere stage was observed with any certainty.

The earliest certain indication of segmentation of the body is in the appearance of a slight transverse constriction in its anterior part, the body being now broadly oval in shape. This constriction marks the division between the peristomium and the next following segment.

The appearance of the first pair of parapodia was not noted. Stages with the chætigerous sacs of three segments well formed were abundant, and in many of the specimens none of the chætæ had yet reached the surface. In this stage cœlomic pouches are observable; but these correspond with the segments only on the left side. On the right side one pouch is often large;

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and sometimes only two are observable. An appearance as of a dorsal blood-vessel with lateral branches is, in prepared specimens, very constant at this stage; but it seems unlikely that such a vessel would yet be established (fig. 2). I did not observe this appearance in living specimens. Two eyes, reddish in colour, are noticeable. There is no trace of tentacles or of anal appendages. A lateral view of a prepared specimen at this stage shows an opening behind each chætigerous sac: these openings are probably nephridiopores.

When four segments follow the peristomium a pair of palps is developed, and the rudiments of a pair of tentacles and of a pair of anal cerci appear (fig. 3). There is no indication of an unpaired tentacle. The notopodium and neuropodium of the appendages of the first pair, those of the peristomium, lose their chætæ and develop as jointed organs with a few rigid hairs. These are to be the peristomial tentacles. In connection with the appendages of each pair is now a pair of flagella. In the case of the peristomium these appear to represent the notopodial cirrus; in the case of the other appendages, the neuropodial cirrus. The egg membrance has by this time disappeared, and the gelatinous matter in the neighbourhood of the larva deliquesces. In this liquid area, which extends constantly, the larva swims by means of its flagella. I am not certain when the flagella disappear; but I could not detect them in any larva that had developed eight segments. Two pairs of small eyes have appeared on the peristomium. In this stage also, that in which four complete segments follow the peristomium, stomodæum and proctodæum appear to develop.

A little later the jaws are observable, and still later they become very noticeable and may be seen to be carried forward with the pharynx, snapping vigorously. The eyes on the prostomium, the first pair that appeared, usually disappear by the time six segments are developed.

Just as the parts of the parapodia of the peristome lose their chætæ, so does the notopodium of the next segment lose its chætæ and develop as a jointed, tactile organ. I saw no change in the neuropodium of this segment (figs. 5 and 6).

I observed the development of several specimens up to thirteen segments; but beyond that time I was unable to keep them alive.

Explanation of Plate XXIII.

Fig. 1. Egg-mass; × 1 ½.

Fig. 2. Dorsal view of larva with three pairs of chætigerous appendages. Cœlomic pouches developing.

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Fig. 3. Another larva in which the first pair of appendages, those of the peristome, have lost their chætæ, and are developing as tentacles. Prostomial tentacles and anal cerci appearing. Flagella present. Two pairs of eyes on the peristomium in addition to the pair on the prostomium. Length, 0.98 mm.

Fig. 4. An older larva. Length, 1.08 mm.

Fig. 5. Ventral surface of head of specimen figured in fig. 4, but drawn twenty-four hours later. In the second segment the notopodium is tentacle-like, while the neuropodium still bears chætæ.

Fig. 6. Dorsal view of the head of the same specimen after another twentyfour hours. The prostomial eyes have disappeared.

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