Comprehensive DescriptionRead full entry
"The history of the genus Atolla is typical, for the bathypelagic group of medusae, for as one deep sea expedition after another brought battered specimens to light, nine species, supposedly distinct, were proposed. But as greater numbers of specimens were studied, the number of these species was successively reduced, until it now seems likely, says Browne (1916), that all the Atollas which have yet been seen, represent variants, or contraction-phases, of two species only; A. wyvillei (the earliest named), with smooth lappets, and A. chuni, in which each of the lappets is studded with 7-9 small papillae.4 Even the sculpturing of the central lens; whether with or without radial furrows at its margin, with the degree of development and exact confirmation of these when present, is found, by Browne (1916), to be less reliable as a systematic character than previously supposed. And this conclusion is supported by the present series, which shows intermediates between the bairdii type, with smooth central lens; the verrillii type in which the lens is scored with narrow radial furrows; and the wyvillei type where at least the edge of the lens is indented with broad radial notches though its central portion may be smooth.
This character, then, goes into the same discard with most of the others which have been used to separate "species" of Atolla; relative breadth of the septal nodes for example, and the number of antimeres.
It was long ago observed (Fewkes 1886; Maas 1897) that Atollas, when taken, or after preservation, show a smooth zone between ring furrow and tentacular pedalia, which is not visible on surface views of other specimens. This difference, according to Vanhöffen (1902) represents merely a contraction- or compression-phase, for none of the Valdivia specimens of Atolla lacked this zone intermediate between coronal furrow and padalia, though in some it was concealed by the everhanging margin of the central lens. Mayer (1910) has, however proposed the presence or absence of this zone anew, as a specific character, dividing the Atollas with smooth lappets into bairdii with it: and wyvillei lacking it. This supposed difference has not been discussed by writers, and therefore demands a word of comment here.
No special attention was payed to this character in my study of the Albatross Atollas from the Eastern Tropical and Northwestern Pacific, beyond the fact that in most of them this zone was concealed (Bigelow 1909, Pl. 9, Fig. 3, Pl. 10, Fig. 9). Reexamination of some of these specimens gives the following results—Out of 8 specimens, in fair condition, from off Japan ('Eastern Sea,' Alba-1ross Station 4907), 30-65 mm. in diameter, seven have the central lens overhanging, and nearly entirely concealing the anular zone, while in one specimen the central lens is much more elevated, leaving the edge of this zone visible, in aboral view.
Two out of three excellent specimens from Bering Sea, Albatross Station 4766, with disc very flat and firm, show no distinct intermediate zone, in aboral view, their pedalia seemingly separated from the margin of the central lens only by the coronal furrow. The third, in equally good state, shows an intermediate zone much narrower than the length of the tentacular pedalia.
As a radial section though the bell of this last specimen shows, the coronal furrow is very deep when this intermediate zone is nearly or quite concealed: deeper, in this case than the length of the tentacular pedalia. And the connecting zone below it, between central lens and peripheral zone, is narrow. Another much flattened specimen, also from Bering Sea (Albatorss Station 4773), about 35 mm. in diameter, also has the central lens practically in contact with the pedalia. And among the Atollas which I have examined such has been the condition in general, in the specimens in which the gelatinous substance was firmest, the bells flattest, and the pigment most nearly intact after preservation, i.e. in those that were probably alive when placed in the preservative, and so may be supposed to have suffered muscular contraction. This same state is shown by Vanhöffen (1902, p. 5, Fig. 24).
Two other specimens from this same station, respectively of 35 and 45 mm., with the central part of the disc more elevated, and the gelatinous substance softer, show intermediate zones between ring furrow and pedalia, nearly as wide as the latter.
The great majority of the large Arcturus specimens, listed below, illustrate a still more relaxed state, with the whole medusa softer and more cup shaped, the central lens bulging higher above the level of the ring furrow, the marginal zone hanging downward, and the smooth intermediate zone about as broad as the tentacular pedalia are long: in some cases even broader.
Radial sections of specimens of this type show the basal connecting zone at the bottom of the coronal furrow, between periphery (corona) and central disc, relatively thicker (1/3 to 1/2 as thick as the coronal furrow is deep) than is the case when the smooth annular zone is less prominent; furthermore, in such specimens the relaxed annular zone is much softer in texture than the pedal zone.
As has long been known, this connecting zone is the site of a circumferential strand of contractile fibres; and complex series of such fibres are imbedded in the gelatinous substance of Atolla.
In short, the apparent presence or absence of the smooth annular zone, has no genetic significance in Atolla. Every gradation is to be found (among specimens in different state of contraction or relaxation) from those in which no such band can be distinguished, other than the smooth outer wall of the coronal furrow, to others showing a protruberant band of this sort, as broad as the pedalia are long and which may or may not be marked off from the latter by a furrow.
Such differences do not indicate the existence of varieties, but reflect nothing more than the state of the animal—whether still alive, or dead and flaccid—when placed in the preservative, or the amount of muscular contraction it may have experienced, in the latter."