Page images
PDF

It seems reasonable to suppose that the anterior part of the worm regenerates a new posterior end after the swarm. Many regenerating worms of all sizes are found, and, indeed, I had two worms whose posterior ends were broken off on June 25, and maintained them alive within coral rocks kept beneath low tide level until July 28, and during this period they had regenerated slender, tapering posterior ends about 4.5 mm. long (see Figure 6).

Experiments on Fertilization, Etc.

When rocks containing the Atlantic Palolo are cracked open the worm begins to crawl out backward, but when about half of the length of the worm has emerged, the anterior part attempts to return into the burrow while the posterior part struggles to swim backward. A constriction appears between the two ends, and the posterior part of the worm is set free and swims through the water, while the anterior crawls back into the rock. In large worms with well developed sexual segments the break is apt to appear exactly at the junction between the sexual segments and the non-sexual anterior part of the worm. The sexual segments then swim rapidly backward and upward to the surface, where they continue to swim backward with great activity. Soon, however, they begin to contract violently, casting sperm or ova 6ut into the water very much as in a normal swarm. It appears that the shock given to the worm upon cracking the rock acts as an abnormal stimulus to produce the drama of the breeding-swarm, which can be produced in this manner at any time of the day. On July 12, 17, 19 and 23, 1902, I obtained eggs and sperm from worms which had thus been induced to go through with the actions of a swarm, but although the sperm was very active, and appeared to be normally attracted by the eggs, not one living embryo was obtained, although many of the eggs segmented abnormally. It was remarkable that eggs obtained at 4 P.M. on July 23 could not be fertilized although less than 12 hours afterwards the normal swarm occurred in which practically every egg was fertilized. This is clue to the fact that none of the eggs become mature until the time of the normal swarm when all of the eggs mature simultaneously. Full-grown female worms which had discharged some eggs were killed on July 12 and 23, at 4.30 P.M., in a saturated solution of corrosive sublimate in 35% alcohol; and four females which were swimming in the swarm at 4.20 A.M. on July 24 were similarly treated. Figure A shows the condition of the eggs in the females killed on July 12 and 23. The nucleus is central in position, the nuclear membrane is complete, and there is a large nucleolus. Figure B, on the other hand, shows the condition of the eggs within the females killed during the swarm at 4.20 A.M. on July 24. The eggs are now about to give off the first polar globule. The nucleus has migrated to the surface at the animal pole, the nuclear membrane has disappeared, the nucleolus has broken up into small spherical fragments,* and the first polar globule is not yet constricted off from the egg.

[graphic][ocr errors]

I hoped to discover whether the Atlantic Palolo responds in its ■ swarm to the light, or to some attraction due solely to the position of the moon. Accordingly a number of worms were maintained alive within coral rocks beneath low tide level, and on the nights of July 23-28, 1902, the rocks containing the worms were placed in glass aquaria which were shut up within a wooden boat shed into which no direct moonlight could penetrate. None of these worms swarmed, but upon cracking open the rocks I discovered that all of the worms were immature. The experiment is therefore inconclusive and should be repeated.

In 1900 I advanced the view that the Palolo may be regarded as an animal in which the egg-laying season has been reduced to a period of one or two days; and that it could be shown mathematically that a shortening of the egg-laying period causes a greater concentration of breeding individuals and therefore shortens the average distance that the spermatozoa must travel in order to fertilize the

*Xot shown in this individual section.

ova. This advantage would allow of an increase in the relative number of females and a reduction in the number of males, and thus more eggs would be produced. The short breeding season of the Palolo may, therefore, have been brought about through the agency of natural selection, provided the ancestors of the Palolos now living possessed an initial tendency to swarm at or near the period of the moon's last quarter rather than at other times, and that this tendency was of a sort that would be perpetuated by inheritance.

Friedlaender, 1901, combats this view, holding that the Darwinian explanation is inadequate. He maintains that the Palolo responds to a certain condition of the tide in conjunction with an influence which the moon exerts upon the worm when at the position of last quarter, and that the response of the worm has not been brought about through natural selection.

It is interesting to observe, however, that the condition of the tide at the Tortugas, Florida, upon the day of the last quarter of the July moon is quite different from its condition at Samoa and Fiji upon the days of the last quarter of the October and November moons. At these times in Fiji and Samoa the tide is about at its lowest when the sun rises, while at the Dry Tortugas the tide is ebbing but is still about l*4"2?4 hours high. For example: On July 27, 1902, at the Dry Tortugas low tide occurred 1 hour and 22 minutes after sunrise, while at Apia, Samoa, on October 23 and November 21, 1902, low tide came at 1 and at 14 minutes after sunrise on the respective dates. Also the mean rise and fall of the tide at the Dry Tortugas is only 1.1 feet, while at Samoa and Fiji it is 2.2—3.7 feet. All that we can say is that Atlantic and Pacific Palolos swarm upon or near the day of the last quarter of the moon, at corresponding seasons of the year, for the meteorological conditions and temperature of the water at the Tortugas in July are remarkably similar to those at Samoa and Fiji in October and November. The worm no doubt responds to some physical stimulus which is dependent upon the condition or position of the moon, but the exact nature of this stimulus remains to be discovered.

Deseription of the Mature Worm.

A minute description of Eunice fncata is unnecessary, as the worm has been well described by Ehlers, 1887; Mem. Mus. Comp. Zool. at Harvard Coll., Vol. XV, p. 91, PI. 25. Ehlers, however, had access only to alcoholic specimens, and a few details may be added from an inspection of living worms. The entire worm is shown in Fig. 1, the figure being of natural size and drawn from a living male worm on June 23, 1902. The posterior sexual segments are not yet fully developed and are destined to become about twice as long and about one-half again as wide as in the figure. Figure 2 gives a dorsal, and Figures 3 and 4 ventral and lateral views of the head, magnified four times. The praestomium exhibits a longitudinal dorsoventral cleft dividing it into two lateral lips, each of which bears a pad-like thickening on its ventral surface. The dorsal sides of the first segment bear five antennae, one median and four lateral. The eyes are large and are situated between the two lateral pairs of antennae. The segment back of the head bears no appendages, while the following segment bears only a pair of dorsal cirri. In full-grown worms the four succeeding segments are provided with lateral parapodia which lack gills. In younger worms there may be five or six of these appendages without gills. The gills are, however, found upon all of the following parapodia of the body excepting those of the sexual segments. The gills of the anterior third of the body are the best developed, and are provided with 7-10 simple lateral filaments. These filaments become reduced in number back of the first third of the worm's length and in the middle of the body the gills have no lateral filaments and are reduced to mere stumps. They disappear entirely at the point of junction of the body segments with the posterior sexual part of the worm, so that the sexual segments are not provided with gills. The external surface of the gills is covered with active cilia. The hindermost segment of the body bears a pair of ventral cirri. It is interesting to observe that the young larva bears a pair of dorsal as well as a pair of ventral cirri upon its posterior segment. As the adult worms never possess these dorsal cirri they must degenerate, leaving only the ventral pair. The anterior third of the worm is of a rich bronze-brown with iridescent color playing over it. The mid-region of the body is duller in color being brown or brown-red. The sexual segments are pink or dull salmon-yellow in the male while in the female they are dull sage-green or dull brownish-yellow. These color differences are. however, due entirely to the color of the contained sexual elements, for when these are discharged the sexual segments of both sexes become dull brick-red in color. The gills are of a rich purple-red color, while the parapodia are light greenish-yellow. The worm is very active in captivity and when disturbed will often protrude its jaws and snap viciously. Full-grown worms are about 350 mm. long. The anterior non-sexual part consists of about 225 segments and the posterior sexual part of about 150 segments.

Summary.

The "Atlantic Palolo" is Eunice fucata, Ehlers. It is found at the Dry Tortugas, Florida, and lives within disintegrating coral rock or coquina from below low tide level to a depth of at least six fathoms. Its breeding habits are closely similar to those of the well-known Pacific Palolo worm (Eunice viridis).

The Atlantic Palolo swarms at the surface before sunrise within three days of the day of the last quarter of the moon between June 29-July 28. The posterior, sexually mature end of the worm breaks away from the anterior end, and swims backwards and upwards to the surface where it continues to swim backward with great rapidity until about the time of sunrise, when it contracts, casting the genital products out into the water. The anterior part of the worm remains below in the coral rock. The worm requires at least two years to attain sexual maturity. There are 57% of males and 43% of females. Only sexually mature worms cast off their posterior ends at the time of the swarm. The immature worms are about twelve times as numerous as the mature.

The shock produced by cracking the coral rock acts as a stimulus to produce the drama of the breeding-swarm before the normal date of the swarm. Eggs obtained in this manner are immature and can- | not be fertilized, even twelve hours before the time of the normal/ swarm. All of the eggs mature simultaneously at the time of the^ normal swarm.

The normally laid eggs float in the water and begin to segment soon after extrusion from the worm. The segmentation is total and unequal, the gastrula is formed by epibole, and the larva is telotrochal. The young larvae swim near the.surface, but sink to the bottom uponv attaining four pairs of setigerous lobes. The posterior segment o" the larva bears a pair of dorsal as well as a pair of ventral cirri. Only the ventral pair of cirri persist in the fully developed worm.

Museum Of The Brooklyn Institute Of Arts And Sciences.

August, 1902.

« PreviousContinue »