Notes on the Breeding Habits and the Early Development of Dolichoglossus otagoensis Benham.

By H. B. Kirk, Victoria University College, Wellington.

[Read before the Wellington Philosophical Society, November 24, 1937; received by the Editor, December 22, 1937; issued separately, June, 1938.]

Dolichoglossus otagoensis is found in considerable numbers at several points on the coast in the neighbourhood of Wellington. The worm never burrows in sand but lives among closely-growing seaweed or in crevices on the under side of stones. It often occurs in sheltered rock-pools in which there is an abundance of coralline seaweeds. These masses of coralline form a veritable nursery and home for many forms of life, and there is an abundance of minute organisms suitable as food for those animals that, like Dolichoglossus, capture their food by means of ciliary action.

Breeding begins about the middle of July and extends to about the middle of August. It has, so far as I have observed, no connection with the phase of the moon. A female worm about to breed seeks a sheltered crevice or a space among the bases of the seaweed stalks and there surrounds herself with a cocoon of sticky mucus, which later becomes tough and stringy, and sand grains and other particles may adhere to it. In most cases the cocoon completely conceals the worm, forming a tunnel open at both ends. Whether fertilisation takes place before the spinning of the cocoon I am not sure. The sperms of the male are freed from the gonad in rounded spermatophoric masses, which roll along slowly. I once found such a mass within a cocoon in which was a female with early embryos, but the sperms of the mass were not motile and could not be induced to display any activity. I never have found a female within a cocoon with eggs that had not at least begun segmentation.

Within the cocoon, the female rubs off the embryos, and cocoons may be found from which the worm has withdrawn leaving a double row of embryos that have not yet completed the first cleavage. This, however, is not an ordinary occurrence. More often it happens that the embryos are rubbed off later, and with them a considerable part of the gonadial ridges. In some cases the worm remains within the cocoon in this mutilated condition. More often it withdraws: sometimes it breaks across and the two portions withdraw. When this happens the anterior portion remains active for some time and undergoes regeneration, although I cannot say whether this ever is complete, but I believe it is. I have not been able to keep the animals alive long enough to be sure. The portions that have been thrown off remain within the cocoon, with the embryos more or less embedded in them. They lose brightness of colour, become very brittle and presently fall to pieces at a touch. It is extraordinary that they do not seem to support swarms of putrefactive organisms.

A very striking result of the manner of shedding the embryos that has just been referred to is that you often find in a cocoon a double line of embryos from six to thirteen in each line, all in approximately the same stage of development.

External Features of Development. Entry of the sperm has not been observed. The fertilised egg is spherical and is surrounded by a perfectly transparent egg capsule, which comes to stand away from the egg as in the case of the fertilised egg of many echinoderms. The first two cleavages are meridional, and the third is slightly nearer to the animal than to the vegetal pole. Cleavage goes on as regularly as in Amphioxus in most cases, and results in a spherical blastula. The difference in size between megameres and micromeres is noticeable, but not more so than in the species described by Bateson (Q.J.M.S., xciv, Apl. 1884). Indeed, Bateson's figures of the blastula might almost apply to this animal. The most noticeable differences are that there is no special “transverse” band of cilia as the blastula begins to invaginate, and that the antero-posterior flattening of the embryo at this stage is here much less marked. As invagination advances ciliation is observable, the cilia developing uniformly over the whole surface. Invagination is perfectly regular. As the blastopore becomes smaller it begins to lose its circular outline and to become flattened from side to side. There is even a suggestion that it will close in the middle of its length, leaving the two ends open. This, however, does not happen, and closure is complete. As the blastopore closes ciliary action becomes more pronounced. The embryo becomes oval and slight movements of contraction are observable.

So far the embryo has remained within the egg capsule, but this has gradually become thinner and at about this stage the embryo escapes. There now begin two changes that alter the shape of the embryo considerably. The first is a slight dorso-ventral flattening, the second a narrowing at the anterior end, the end farthest from the position formerly occupied by the blastopore. The two collar grooves appear in their order, but before the second groove can be distinguished the longitudinal groove on the proboscis, that is such a marked feature of the adult, makes its appearance. It is for some time well marked on both dorsal and ventral surfaces. Before the collar is completely marked off there appears on its dorsal surface a shallow medullary groove, which later closes. Mouth and anus develop at about this time and the animal glides slowly about by means of its cilia, looking much like a very sluggish planarian. Its length is not twice its breadth, and the part that will form the proboscis is longer than the part that will form the body.

It has been stated that there is no post-oral band of cilia. Neither is there an apical tuft, although there is a loose group of nerve cells seen on longitudinal section, just below the ectoderm at the apex.

It very frequently happens that within the egg capsule there is a body much smaller than the embryo, that grows and segments for some time. In one case this was observed to have reached a size nearly half that of the embryo itself. I could not determine that this was the result of detachment of a blastomere, and, as it may be seen before what seems to be the first cleavage is completed, it probably is one of the polar bodies. It may persist until the gastrula stage is completed. Ultimately it breaks down and disappears.