In the course of my taxonomic studies of the collections of Pycno- gonida in the United States National Museum, the Museum of Com-. Some workers have assigned this genus to the Phoxichilidi-idae, others consider it a member of the Pallenidae.
NYMPHONJDAL
TANYSTYLIDAL
PALLLNIDAL
PYCNOGONIDA — HEDGPETH
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TEN-LEGGED FORMS
James Eights (M.D.) and his Antarctic voyages, see The Reincarnation of James Eights, Antarctic explorer, by John M. There is also an interesting diagram of extraosseous pycnogonids, and a facsimile of the original figure of Decolopodaaustralis.
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It would appear, therefore, that these two forms are roughly similar, with essentially the same range of variation, and that the impossible specimen of the absence of the posterior segments of the "genus" would be impossible. Although it may be objected that the genus Nymphon is so large and complex that it would be easy to find an approximate counterpart of a leggy form, the case is.
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Evidently there is some variation in the construction of these spines with individual specimens, for the differences are too great to be explained by artistic interpretation of a small structure whose. The two species are remarkably similar in general appearance and shape of the chelae. However, if it were twice as large, its position on the chart would be exactly where expected, namely in the same relation to Pentanymphon antarcticxim as NympJion gracillimtini is to A^.
Therefore, it can be assumed that the trend of the leg-trunk ratio of Pentanymphon minutum is parallel to that of the other three forms. It is interesting to note that this Heard Island specimen, which is the closest record for the type Decolopodato and only known locality for Dodecolopoda, shows more similarities.
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As Caiman and Gordon (1932, p. no) pointed out, the occurrence of a pycnogonid with six stem somites "does not really involve any important modification of the problem presented by the ten-legged species." As Table 3 shows, the greatest divergence between Dodeco-. Unlike the decapodous nymphs, in which body length increases substantially with the addition of a fifth somite, Decolopoda and Dodecolopoda appear to undergo no appreciable increase in body length with the addition of somites relative to the ratio of the closely related C.
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A further coincidence is that the leg-to-trunk ratio curve of the Nymphon-Pentanymphon group is also inclined at 30°, although it shows a downward rather than an upward trend. It is easy to be baffled by these beautiful graphs, and they have so fascinated several biologists that their contributions on the subject could be called a logarithmic analysis of the Adyo. Although Pcntacolossendeis reticulata is not a rare species, and can probably be collected almost at will along a hundred fathoms.
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NO. I( PYCNOGONIDA HEDGPETH
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It is interesting to note that the angle of the third pair of lateral processes in this specimen is approximately average. This intermediate plate is so placed that it balances the second and third legs of the normal. Ohshima (1942b) suggested that the aberrant specimen of Nymphonella tapetis may have been formed by the failure of the limb buds to divide, but, as he predicted, this would not explain a similar abnormality in a form in which the legs were not formed in this way.
On the other hand, instability in the last stem segment is suggested by Ohshima's six-legged specimen, and he (1942a, p. 260) is of the opinion that the pentameric forms arose through such instability: "Thus whether the increase or decrease in the number of body segments, and consequently in appendages, occurs at the junction of the trunk and the tail (abdomen), but not as hypertrophy or abortion that occurs at the morphological rear of the body." Even if this could be answered satisfactorily, the question still remains: what exactly is the nature, the cause and significance of the decapodic condition in the Pycnogonida. It must also be pointed out that this phenomenon of reduplicated segmentation, or polymerism, occurs in those branches of the Pycnogonida which, on account of their large numbers of closely separated and numerically abundant species, may be regarded as the most successful of the evolutionary ones. point of view
10For a discussion of the dynamics of evolution, see Lotka, Alfred O., Elements of Physical Biology (Williams & Wilkins, Baltimore, 1925), chap.
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Such laboratory investigations may show that metamericre-duplication among the Pycnogonida is an entirely different type of variation than hitherto known, but whatever its mechanism, the fact remains that it is too closely related to particular species to be a random coincidence or genetic accident. It can be found that the basic chromosome pattern of the three families in which he. The success of this variation, as shown by its relative abundance, shows that it is in some way advantageous, though we may not be able to understand where the advantage lies.
Tate Regan, who participated in a discussion of this problem at a meeting of the Linnean Society, noted that Dodecolopoda and the decapodous forms seem to be an example of "evolution by accident, a phenomenon difficult to understand."^^ Possibly he has in. William Morton Wheeler would have considered the polymer Pycnogonida an example of emergence, which it was. For a statement of Wheeler's position, see Emergent evolution and the development of societies, in Essays in Philosophical Biology, pp.
For the current philosophical status of the concept, see Irwin Edman's introduction to the Modern Library edition of Bergson's Creative evolution.
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Of course it is dangerous to argue by analogy from the human mind, but the basic instinct of all great minds, be they scientists or philosophers, theologians or poets, to achieve unity from the multiplicity of things we know and perceive, suggests that Nature is built into the same endless adaptations of the different but essentially similar are shaped by the demands of the external environment.^^. Although the observation of Marcus (1940a, p. 197) that "the active and passive means of distribution of Pycnogonida appear to be less than those of all thermomarine arthropods" is essentially true of marine species, there are several noteworthy examples of extended distribution that is difficult to explain, and future collections, especially of smaller forms, may prove that many apparently local species are widely distributed. Marcus' suggestion (idem) that the relatively low mobility of the Pycnogonida "may in some cases have encouraged the development of a large number of well-separated species," for most coastal species are probably confined to relatively small areas.
This coastal species is easily identified by the absence of the terminal claws on the legs, and all workers who have identified it from widely distributed parts of the world have been willing enough to provide adequate figures. Therefore, there can be little doubt that all data, such as widespread as they are, represent the same species. Another widely distributed coastal species is Achelia echinata, which has been identified in northern Europe, the Bay of Naples, the Atlantic coast of Morocco, San Francisco, southern Alaska and the Aleutian Islands, Japan, the Siberian coast near Vladivostok and Kiaochow. China. Furthermore, the uniform character of the specimens from different sites suggests that this is a young species.
Benedict, Government Naturalist for the Falkland Islands, is interesting: "I have taken the Caprella with a trawl in about the middle of the Atlantic.
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This may indicate that their appearance in the western Atlantic precedes their distribution on the eastern shores of the ocean. It is possible that this is more apparent than real, for collections from the north coast of South America and the West Indies are very inadequate, and several species described from Brazil have appeared in collections from the West Indies.-^. There is only one genus endemic to the North Atlantic, Paranymphon, and it is a deep water, not a littoral -^See Hedgpeth, 1943b.
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One of the most striking differences is the absence of Tanystylmii in Japan and the northwest Pacific as a whole, although several species occur along the California coast. This difference between the fauna of the western and eastern shores of the Pacific Ocean can be partly explained by the lack of a convenient bridge of floating sargassum, such as exists in the northern Pacific Ocean. The intrusion of large amounts of Arctic water south of the Aleutian chain is.
Fifty or sixty years ago the bipolar hypothesis received much attention, but D'Arcy Thompson gave it a rough treatment, pointing out that the theory was built on an inadequate systematic basis.-" Now the bipolar hypothesis, so far as the Arctic and Ant-26 A comprehensive comparison between the Japanese and California coasts, see Gislen, T., Physiographic and ecological investigations relating to the littoral of the North Pacific.
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There are also two other genera which are characteristic members of the Cape fauna (Discoarachne and Hannonia), but both of these have been identified from Port Natal. It should be noted that four of the southern hemisphere genera occur along the South African coast. This concentration is not surprising given Ekman's (op. cit., p. 275) summary of endemic forms from this region.
Although there are many small genera in scattered parts of the tropical belt, there is only one large genus, Anoplodactyhis, which could be considered typically tropical. While it is represented by several species in temperate latitudes, it reaches its greatest speciation in the tropics, especially in the West Indies. In general, the endemic genera of the Southern Hemisphere appear to be more widespread than those of the Northern, which is not surprising given the more open character of the Southern Oceans.
Each successive Antarctic expedition will establish the circumpolar distribution of more Antarctic species, and coastal collections on the South Sea Islands will undoubtedly reveal many.
SUMMARY
This is Pycnogonuui, and its large number of endemic species may be due to the heavy body shape and sluggish movements characteristic of the genus. A close relationship between the fauna of South Africa and the East Indies has already been noted by Flynn (1928, p. 3), who suggests that "the great equatorial current is responsible for such phenomena."
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BIBLIOGRAPHY
PYCNOGONIDA — HEDGPETH 43
1943b. Pycnogonida of the West Indies and South America collected by the Atlantis and earlier expeditions.
PYCNOGONIDA — HEDGPETH 45
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PYCNOGONIDA — HEDGPETH 47 TOPSENT, E
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NO. U PYCNOGONIDA — HEDGPETH 51
