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Introduction
Two soil samples collected by Mr. A. C. S. Wright from the floor of the main crater on Raoul Island, in the Kermadec Group, yielded rich cultures of two microscopic aquatic oligochaetes. The species are cosmopolitan in distribution, but this occurrence is of interest in that they have not previously been recorded from the South Pacific area, nor have they been recorded previously in soils.
Their presence in two of the soil types (Tui silt loam and Green Lake loamy fine sand) of the floor of the crater is probably correlated with the existence of an old crater lake. There are abundant signs that a lake once filled the south-west part of the main crater, resulting from a minor eruption on the present site of Green Lake which ejected material blocking the natural drainage channels from the western and southern slopes of the internal watershed of the main crater (Fig. 1) Although normally found in aquatic environments, these worms can withstand dry conditions by encystment (Beddard, 1892a).
The two species described belong to the genus Aeolosoma Ehrenberg (1831), principal genus of the family Aeolosomatidae. The group has recently been described by Marcus (1944), who gives a key to the species. One of the Raoul Island species resembles Aeolosoma kashyapi Stephenson. The second species appears to be identical with A. niveum Leydig, for which no adequate descriptions have been previously given (Beddard, 1895; Michaelson, 1900). Marcus (1944) refers to a description by Ude (1929), but I have not been able to consult this source.
Aeolosoma kashyapi Stephenson, 1923. (Figs. 2a. 2b, 4)
The fully grown worm is about 0.8 to 1.0 mm. in length, with a broad ciliated prostomium, the activity of which aids the animal in ingestion and movement. There are up to 12 segments with a division between the seventh and eighth which marks the budding zone. n, the number of segments of the adult worm, is therefore seven and there is never more than the one daughter worm.
The prostomium is flattened and almost circular, being about 80μ in diameter. Ventrally it is wholly ciliated, the beat of the cilia directing debris and food particles to the buccal funnel, which is also strongly ciliated, as is the whole of the alimentary canal. The edge of the prostomium is marked by a non-ciliated rim, about 5μ in width, which adheres to the surface of the substrate. The outer margin of this rim is also ciliated. The peristomium, the first true segment of the worm, appears circular when viewed from the ventral surface. Ventrally the opening of the buccal funnel occupies most of the segment. It is surrounded by a broad wall, about 10–12μ in width, which does not meet anteriorly but fuses with the prostomium, forming a channel through which the food particles are driven by the cilia of the prostomium to the buccal funnel.
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Fig. 2—(a) Acolosoma kashyapi. Ventral aspect, showing internal anatoms. (b) A. kashyapi. Nephridium.
Fig. 3—(a) Acolosoma niveum. Ventral aspect. (b) A. niveum Nephridium.
Fig. 4—Typical seta.
b.f., buccal funnel, br., brain, b. v., vential blood vessel; b. z, budding zone, e. g., colour gland. int., intestine; n., nephridium; nst., nephrostome; oe., oesophagus: pr. prostomium. s. seta: st., stomach.
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The alimentary canal is characterised by a sinuous oesophagus which extends to the fourth segment, a broad stomach from the fourth to the sixth segment, and a narrow intestine which runs from the seventh segment to the end of the body. There are no well-developed inter-segmental septa, but the gut is attached by a number of bridges to the body wall. There is typically only one such bridge per segment, starting wtih the fourth segment. Between the seventh and eighth segments, where budding takes place, the gut and body wall are much more closely apposed, there being a definite union of connective tissue for the greater part of the zone. The anus is quite simple and terminal. Ciliary action suggests that it may be an auxiliary respiratory organ. The gut shows typical peristalsis.
Nephridia are present on both sides in segments IV to VII inclusive and in segment IX. The nephridium is a simple tube with a ciliated nephrostome coiled in the form of a loop (Fig. 2b), the halves of which are closely apposed and intimately connected, so giving the appearance of an oval body, 30 × 15μ in size. There may be more than one loop, but the oval shape is retained and is very characteristic. There is a non-contractile ventral blood vessel formed by the union of two circum-buccal vessels and a dorsal contractile vessel anteriorly. The brain was the only part of the nervous system observed. This organ, somewhat kidney-shaped and formed by the partial fusion of the two cerebral ganglia, can be observed from the ventral surface as lying in the posterior half of the prostomium. The posterior indentation of the brain is conspicuous, but the organ varies slightly in shape with different individuals. The integument is about 5μ in thickness and is conspicuous by the presence of numerous colour glands. These are also present in the prostomium and are deep orange. These colour glands are the most striking feature of the worm and form a striking contrast to the drab colour of glandless worms. The setae are of importance in establishing the specific status of the material They are embedded in a sac which projects from the body wall into the coelomic cavity, and to the tip of the sac protractor and retractor muscles are attached. The contraction of these muscles enables the setae to serve in the locomotion of the animal, supplementing the contractions of the body musculature.
The setae are all capilliform, fine and hair-like, and about 80–85μ long in the adult worm (Fig. 4.) They are identical in both the dorsal and ventral bundles. The number of setae in each bundle varies, there being up to 5 in the dorsal and up to 3 in the ventral. The third segment, however, has 4 setae ventrally and not 3 in the mature worm. The segments of the daughter bud have only 2 setac in both dorsal and ventral bundles.
No clitellum was observed; nor were any reproductive organs. Fission was the only observed mode of reproduction. In the material observed only one bud appeared to be formed at a time. The limitation of size to only 12 segments would, of course, only allow one daughter bud to be lormed at a time. The budding is quite typical, the thickening between the segments VII and VIII giving rise first to a slight protuberance, the join narrowing and finally being nipped off and the daughter bud then developing the elaborate prostomium of the adult and its full complement of setae.
Aeolosoma niveum Leydig (1865). (Figs 3a, 3b, 4.)
This worm, which is about 1 mm. in length, has up to 10 segments; the broad, flattened prostomium typical of the genus; the wide buccal cavity; and capilliform setae. The budding zone is again between segments VII and VIII; n is 7. The prostomium is flattened and completely eiliated ventrally, as is the outer rim of the prostomium. The peristomium surrounds the wide buccal cavity, and
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like the prostomium is somewhat broader than the rest of the body. It has the same broad wall and surrounding rim as the previous species. The cerebral ganglion is strongly indentated, showing the partial fusion of the supraoesophageal ganglia.
The nephridia, in distinction to those of the previous species, are not in the form of a loop. They consist of a simple tube with a ciliated nephrostome as in the previous species (Fig. 3b), but this lies and winds along the side of the gut. The ciliated nephrostome is somewhat bulb-like and the tube swells again at its posterior extremity. The setae are of the same kind and size in both dorsal and ventral bundles. They are fine, hair-like and capillary with a slight bend as in the previous species. They are up to 65μ in length. (Fig. 4.) Unlike the previous species there are the same number in both dorsal and ventral bundles of each segment. The maximum number is 4 which are present in III; the other segments have a maximum of 3 excepting the posterior daughter segments, which have only 2, as in the previous species.
There are no colour-glands in the integument and consequently this worm is colourless in distinction to the preceding species.
No reproductive organs were observed, multiplication by fission being the invariable rule.