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"Of this species only very few specimens are known. The Valdivia Expedition (Vanhoeffen, 1903, p. 12) caught two beautiful individuals at St. 120 between the Cape of Good Hope and Bouvet Island in 42° 18' S, 14° 1' E in a vertical haul at 1500 m. (821 fms.); the 'Scotia' (Browne, 1909, pp. 240–I) took one damaged specimen between South Georgia and Bouvet Island at St. 450, 48° S, 9° 5o' W in 1332 fms. Vanhoeffen (1903) gives two large coloured figures (general shape) of this species in pl. i, figs. 1 and 2, and a small one representing the lappets with the warts in pl. v, fig. 26. He gives no description and confines himself to mentioning a few characteristics: the highly vaulted bell, twenty-three radial furrows on the central disc, twenty-four pedalia and last but not least the lappets adorned with warts or small papilla-like protuberances. Mayer (1910, p. 562) mentions as a character an annular ridge hidden within the ring furrow, but this ridge is not to be seen in Vanhoeffen's figures, and it is not mentioned in his description.
In the Discovery material no less than forty-three specimens are present, on most of which the warts (‘glass beads’) on the lappets are distinctly visible. In some developmental stages of about 10 mm. diameter and in a few large specimens the warts are very feebly developed. There is a remarkable constancy in respect of the number of tentacles (24), pedalia (23) and radial furrows (23) on the central disc.
SIZE OF THE BELL (Plate XIV, figs. 3,4; Plate XV, fig. 5). The diameter of the largest specimen with mature gonads is 65 mm. The specimens caught by the ‘Valdivia’ had a diameter of 27 and 50 mm. and those taken by the ‘Scotia’ may have had a diameter of about 50 mm. Atolla chuni does not attain the size of A. wyvillei, but is considerably smaller.
The CENTRAL DISC is relatively broad, exceeding half the diameter of the bell. Its surface as a rule is smooth, but sometimes there is to be seen a more or less fine or irregular network of larger or smaller polygonal meshes. The radial furrows on the central disc are mostly very faint and feebly developed according to the verrilli type, or the border of the central disc is plain as in bairdi. In a few specimens only, from St. 391, the furrows are deeper and a little broader, resembling more the wyvillei type.
The HEIGHT OF THE BELL (Plate XIV, fig. 3) is very variable, very flat individuals being found together with relatively high ones. Often the central lenticular disc resembles a highly vaulted watch-glass. On the whole this species seems to be a little higher than wyvillei, and the central disc more vaulted.
The PEDALIA are all as long as broad or a little longer than broad (Figs. 7 a, b), very regular, more or less circular in outline, mostly ball-shaped, forming together a row of regular large round beads, seldom flattening each other, and not so uniformly or so strongly compressed as in the different forms of wyvillei. They are always smooth, without furrows
The MARGINAL LAPPETS are not pointed and vaulted as somewhat schematically figured by Vanhoeffen (1903, pl. v, fig. 26). In young specimens (fig. 7 a) they are broad and spoon-shaped, and lie side by side. In older individuals they are relatively longer, elongated, and have grown together at their bases so as to form pairs and lie slightly one above the other, like scales (Fig. 7 b)
The WARTS ON THE LAPPETS show as a rule the number and arrangement described by Vanhoeffen, but they are larger, much stouter, and do not so much resemble round pearls (‘Glasperlen’), being more like elongated drops. They lose their pigmentation more easily than the deeper lying parts of the lappets and are therefore conspicuous as white transparent spots between the surrounding dark brown pigment. In many cases I could only distinguish seven warts in the way described by Browne (loc. cit.), three on each side, and one in the middle; in these the innermost warts were missing. In other cases there were five on each side, and one in the middle. Sometimes the warts on each side of the rhopalium coalesce to a more or less continuous longitudinal irregular ridge or crista (see Fig. 7 a on the left). Sometimes a few irregularly dispersed ‘pearls’ are found between
the regularly arranged ones, e.g. two median ones. Sometimes a small longitudinal thickening of the exumbrellar jelly is found above the rhopalium itself. A few specimens have eleven warts, four on each side, two in the middle and one above the rhopalium. In several cases the most distal warts near the end of the lappet are the largest and have properly the form of a drop.
The RHOPALIA show no peculiarities except that they are surrounded at the base with a thick dark brown mantle of pigment, and in several cases I have seen more or less distinct traces of pigment in the stalk of the rhopalium itself.
The GASTRO-VASCULAR SYSTEM of Atolla chuni (Figs. 8 a, b) agrees on the whole with the descriptions given by Vanhoeffen (1903) for A. valdiviae, by Maas (1903, 1901.) for bairdi and by Bigelow (1909) for wyvillei, but there are differences enough to distinguish A. chuni from other species of the genus, even with the naked eye. The rhopalar canals and tentacular pockets are here strongly pigmented. The rhopalar canals are thickened locally in their outermost part and become thinner again in the neighbourhood of the rhopalium. Besides this a large subumbrellar papilla lies in the middle above each rhopalar canal. In young specimens (Fig. 8 a) this is shaped like a ball or egg, and in older ones (Fig. 8 b) distally elongated to form a rounded longitudinal ridge, which diminishes in height towards the margin. Beneath this papilla the otherwise dark rhopalar canals are very poor in pigment and become nearly transparent, so that they are visible only by their outlines. The sudden thickening of the rhopalar canals, combined with the bright transparent oval spot formed by the papilla and the dark purple-brown pigmentation around it, gives its own cachet to the gastro-vascular system of A. chuni.
In the tentacular pockets the FALSE SEPTA, as already observed by Maas in A. bairdi and by Bigelow in wyvillei, can almost always be observed. They are very variable in form and size, being long, short, broad, or narrow. There is such great variability of these septal regions subdividing the tentacular canals that they do not prove to be of any systematic value as believed by Maas.
The small and narrow VESSELS IN THE LAPPET POUCHES reach very far into the lappets so that at their distal ends only a small piece of the endoderm lamella is still visible. For comparison I give here two figures of the gastro-vascular system, the first showing that of a young specimen of about 15 mm. diameter (Fig. 8 a) and the second (Fig. 8 b) that of an adult animal of about 50 mm. diameter. The differences in the form of the tentacular pouches in both cases are obvious. In the younger specimen the tentacular pouches are distally very broad. Where the long thin lappets arise their outline is curved inward. In the older stages the tentacular pouches are much more elongated, more slender and distally less expanded, and the incurvation at the origins of the lappet canals is less pronounced. In both cases the pigment of the tentacular pouches reaches to the bases of the tentacles surrounding them. In the older specimens the SUBTENTACULAR POUCHES on the subumbrella near the insertion of the tentacles are indicated in Fig. 8 b with fine dots. The ends of these pouches reach far into the lappets and are also deeply pigmented. This makes it rather difficult to understand the anatomy of the lappet zone and of the insertion of tentacles. A thorough study of the anatomy of these parts seems to be very necessary, but this is beyond the scope of the present report. In both figures the circular muscle is figured as being partially cut off.
The COLOUR is in all specimens nearly the same, the smaller ones generally a little less dark than the older ones. All are a very dark, deep purple-brown, and much more intensely and brilliantly coloured than in the somewhat faded specimens figured by Vanhoeffen. On the exumbrella all grooves or deeper lying parts are deeply pigmented, especially the coronal furrow, or show at least traces of pigment, as, for instance, the radial furrows on the central disc. In most specimens the subumbrella too is more or less covered with a thick layer of the dark purple brown pigment, which has become partially separated, or hangs here and there in small patches from the surface. Often the ring muscle is wholly covered with pigment, or stained in many places with reddish brown spots, as is the case with the gonads. The gastro-vascular system is very distinctly visible, by its own strong pigmentation, through the more superficial thin pigment stratum. The circular muscle and gonads are, if not covered with pigment, of a yellowish colour, and the muscle sometimes has a greenish yellow hue and is more or less iridescent. The stomach, if not covered by dark pigment, has a greyish blue tint. Generally speaking the pigment in the specimens of A. chuni is better preserved than in those of A. wyvillei, and it seems to be darker and more resistant.
The GONADS develop rather late. Specimens smaller than 30 mm. diameter have no gonads. In specimens of 50-60 mm. diameter the gonads are strongly folded and form a nearly complete ring, while in smaller ones they are in the form of eight perfect sacs which lie separated from each other at equal distances, or constitute four pairs. Often it was possible to distinguish the eggs in the sacs with the naked eye. If not mature, the gonads are easily overlooked, because they are covered with the thick layer of dark pigment.
GEOGRAPHICAL DISTRIBUTION (see Fig. 9). The catches of the ‘Discovery’ have greatly enlarged our knowledge of the distribution of Atolla chuni. Up till now only three individuals from two localities in the neighbourhood of Bouvet Island were known. In the broad band between 30 and 55° S the ‘Discovery’ found no less than forty-three specimens. Most individuals have been taken between the west of the Cape of Good Hope and north of South Georgia, and one haul (St. 391) is from the south-east of the Falkland Islands. The greatest number of individuals has been collected at this station (391) with twenty specimens of different sizes, and at another rich haul at St. 395 (north of South Georgia) there were six specimens. In three localities (Sts. 407,72,150) A. chuni has been taken together with A. wyvillei and Periphylla hvacinthina. The distribution of Atolla chuni is wholly restricted to the sub-Antarctic part of the Atlantic Ocean; the medusa has not yet been found in Antarctic waters south of the latitude of Cape Horn, nor to the north of 30° S. Comparing the distribution of A. chuni with that of A. wyvillei in the same regions, we find that in both cases most individuals are gathered in the neighbourhood of continents or groups of Islands.
VERTICAL DISTRIBUTION (see Table IX). A. chuni is a true deep-sea medusa with the typical pigmentation of abyssal forms. The ‘Valdivia’ found it in ± 1500 m. depth, and the ‘Scotia’ in about 2000 m. The ‘Discovery’ never collected A. chuni in closing net hauls from 800 m. upwards. The most superficial catch was made south-west of the Cape between 800 and 900 m. and between the Cape and Bouvet Island in 850-950 m. In all other closing net hauls the species was present only in depths of 1000-1500 m. The specimens collected with open nets were also found only when nets were used at depths exceeding 1100 m. All these facts prove A.chuni to be a true component of Bigelow's so-called intermediate fauna. It prefers deeper layers than A. wyvillei. Most striking is the rich catch from St. 391 (south-east of the Falkland Islands) from 1200 to 1300 m. depth, with twenty-one fair individuals, almost the half of the whole. It is a pity that it was not a closing net haul. In the closing net catch from St. 395 (north-east of South Georgia) six specimens were caught at a depth of 1500-1600 m. The maximum occurrence is therefore in a layer between 1200 and 1600 m.
In both basins of the South Atlantic the species lives in the sub-Antarctic or Antarctic intermediate water—in the so-called ‘Subantarktische Zwischenstrom’ of Wüst . The specimens at Sts. 407, 107, 256 and 76 were taken in this water, which has a low salinity: at these stations it was about 34.35-34.45 parts per thousand. South of about 40° S. A. chuni was found in different water, with a salinity of 34.65-34.75 parts per thousand which flows southwards below the north-flowing Antarctic water. (Here I follow the suggestions given by Mr G. E. R. Deacon.) The previous records of A. chuni (‘Valdivia’ and ‘Scotia’) show a salinity of 34.7 parts per thousand. According to Wüst 's temperature section (1928, pl. xxxiv) A. chuni occurs in a layer of minus temperatures as well as in water from 0 to 10° C."