I04 



NATURE 



[July 28, 1910 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 



expressed by his correspondents. Neither can he undertake 



to return, or to correspond with the writers of, rejected 



manuscripts intended for this or any other part of Nature. 



No notice is taken of anonymous communications.] 

 Antarctic Pycnogons. 



One of the most remarkable and unexpected zoological 

 results of recent explorations in Antarctic seas has 

 been the discovery of Pycnogonida (so-called " sea-spiders ") 

 having five instead of the usual four pairs of legs. Soon 

 after the return of the Discovery expedition, Mr. T. V. 

 Hodgson described Pentanymphon antarcticum, and a 'ittle 

 later he had the good fortune to find, among the collections 

 of the Scottish National Antarctic Expedition, the long- 

 forgotten Dccolopoda australis described by Eights more 

 than seventy years ago. A second species of Decolopoda 

 was added by Prof. E. L. Bouvier from the collections of 

 the French Antarctic Expedition in the Franfais. The 

 most surprising circumstance connected with this remark- 

 able departure from what had been regarded as the normal 

 structure of the Pycnogonida was that it appeared in two 

 genera by no means closely related to each other, but, on 

 the other hand, not dissimilar in general structure from 

 other " normal " genera. Thus Pentanymphon is indis- 

 tinguishable, except in possessing an extra pair of legs, 

 from Nymphon, while Decolopoda is onlv a little more 

 divergent from Colossendeis. From the point of view of 

 phylogeny, two explanations of these conditions may be 

 offered. Most authorities, for example, Prof. Bouvier and 

 Prof- D'Arcy Thompson (in the " Cambridge Natural 

 History "), adopt the view that the ten-legged condition is 

 the primitive one, and has been retained by the most 

 primitive members in two divergent branches of the group. 

 The other e.Kplanation, first suggested by Prof. G. H. 

 Carpenter and advocated by the present writer {Science 

 Progress. April, 1909), is that the decapodous condition is 

 a recent development, appearing independently in the two 

 cases. 



Prof. E. L. Bouvier has just announced (C. R. Acad. Sci., 

 July 4) a very important discovery which shows that the 

 zoological possibilities of the Antarctic seas are far from 

 being exhausted. Among the collections obtained by Dr. 

 •Charcot's recent expedition in the Pourquoi Pas? is a 

 representative of a third genus of decapodous Pycnogons, 

 and, strange to say, it appears to be quite unrelated 

 {withm the limits of the group) to the other two. Penta- 

 pycnon charcoti is a near relative of Pycnogonum, hitherto 

 regarded as the most highly specialised of all Pycnogonida. 

 ''"'■ther, just as Pentanymphon is accompanied by species 

 of Nymphon, and Decolopoda by Colossendeis, so Prof. 

 Bouvier finds that Pycnogonum, hitherto unknown from 

 Antarctic seas, is represented by a new species alongside 

 of Pentapycnon at the South Shetlands. 



Prof. Bouvier regards this discovery as quite in accord- 

 ance with his views as to the evolution of the Pycnogonida. 

 He believes that the group has evolved from a decapodous 

 stage m which at least the chief divisions, represented by 

 Pentanymphon, Decolopoda, and Pentapvcnon, were already 

 ■differentiated from one another, and he supposes that the 

 ■suppression of the posterior legs has occurred independently 

 at least three times with very little accompanving variation 

 m other characters. 



No doubt Prof. Bouvier will justifv these views in greater 

 ■detail when he comes to publish his final report on the 

 Pycnogonida of the Pourquoi Pas? At first sight, how- 

 ever, the discovery of Pentapvcnon would seem to weigh 

 heavily on the side of Prof. Carpenter's hvpothesis. While 

 Pentanymphon is at least as primitive as Nymphon, and 

 even Decolopoda can be admitted, without much difficulty, 

 as a reasonably primitive form, no student of the Pycno- 

 gonida will question that Pycnogonum is one of the most 

 highly specialised members of the group, and the only 

 single character that Prof. Bouvier can find to place Penta- 

 pycnon on a lower level of specialisation is the presence of 

 additional genital apertures, about which he speaks some- 

 what doubtfully. The fact that all three genera are found 

 only in one restricted geographical area is also against the 

 idea of their being survivors of a primitive group, for it 

 NO. 2126, VOL. 84I 



can hardly be supposed that the Pycnogonida underwent 

 practically the whole of their evolution in the Antarctic 

 seas, and only became distributed over the rest of the globe 

 when they had reached nearly the final stages of family, 

 and even generic, differentiation. It seems much more 

 probable (though the like has not yet been suggested for 

 any other arthropods) that some mysterious influence of 

 environment in these Antarctic regions has, so to speak, 

 upset the stability of the octopodous condition and led to 

 I he independent appearance of an additional somite and 

 pair of legs in several unrelated families. 



It is to be hoped that the British expedition now on its 

 way to the Antarctic may obtain material for throwing 

 light on this problem. W. T. Calm.^n. 



A New Italian Orchid. 

 The enclosed photograph represents, I believe, a species 

 of Italian orchid which has not before been discovered. M. 

 H. Correvon, who is, I suppose, the greatest living 

 authority on the European orchids, considers it " seems to 

 be out of the way of species known." When first I found 

 it, I believed it to be Serapias triloba, but, having studied 

 various authorities, I have since come to the conclusion 

 that I was wrong. They all agree that the lip of triloba is 

 crenulated, but in my specimen there is no sign of that 

 Triloba is considered to be a hybrid between Serapias lingua 



and Orchis papilionacea ; although the ground was covered 

 with the former, I could not find a vestige of the latter in 

 the whole of that district. 



.Also, the authorities only give the Riviera as the habitat 

 of triloba, whilst I found my plant in mid-Italy, near 

 Florence. In my plant the sepals and petals are each very 

 distinct and separate, not joined into a cap, as in all the 

 other varieties of Serapias. 



It may possibly be a cross between S. lingua and 

 O laxifolia, but, even so, it differs much from that pic- 

 tured in Barla's book. It is a somewhat striking plant, as 

 the flowers are very large in comparison to the rest, both 

 the labellum sepals and petals being a very vivid shade of 

 colour between pink and purple, the former a little darker. 

 W. Herbert Cox. 



Centre of Gravity of Annual Statistics. 



The principle of taking the centre of gravity of annual 

 rainfall is free from objection, but Mr. Cook's method 

 (Nature, March 31) is mathematically incorrect. Annual 

 statistics should not be plotted on a straight line, but round 

 a circular ring in accordance with the probable etymology 

 of the term (annus, anulus). 



For rainfall in India, results of some value may be 

 obtained by Mr. Cook's method, because practically the 



