Jtuy I, 1880] 



NATURE 



191 



" additional facts not contained in tlie paper " read by him to 

 the Linnean Society. 



Although Prof. Allrnan does not directly allude to my article 

 of the week before (p. 147), I may assume that the statements 

 which he makes in opposition to my conclu'^ion that Limnocodium 

 (Craspedacustes) belongs to the group of the Trachomednsas were 

 elicited by the publication of my results. 



I intend in the July number of the Quarterly Journal of 

 Microscopical Science to show in an illustrated memoir that, con- 

 trary to the conclusion of Prof. AUman, the tentacles of Limno- 

 codium do resemble those of the Trachyline Medusje in their 

 insertion and in the possession of true (thougli radimentary) 

 peronia, as I stated in my original note and in my paper read to 

 the Royal Society on June 17. I shall also show that my state- 

 ment that the so-called lithocysts or marginal bodies have 

 essentially the same structure as those of Trachyline Medusa 

 (being modified tentacles with an endodermal axis) is warranted 

 by the developmental history of the bodies in question. Con- 

 sequently I adhere to my original determination of the affinities 

 of the new Medusa as one of the order Trachomedusfe, and 

 cannot agree with Prof. AUman that its affinity with the Lepto- 

 medusce must be regarded as the closer of the two. 



Prof. AUman slates that he has arranged certain methods of 

 observation with Mr. Sowerby, by which he hopes to determine 

 the developmental history of Limnocodium. It will be of the 

 greatest interest to have this matter fully investigated, and to 

 know what are the methods which Prof. Allman has devised to 

 this end. Mr. Sowerby informs me that at present he has under- 

 taken no experiments of the kind excepting the isolation of 

 specimens in two glass jars in the lily-house, which he carried 

 out at my special request on June 13. 



In the meantime I may say that I have fully satisfied myself 

 that Limnocodium develops directly from the egg. When 

 specimens are kept living in a glass jar under constant observa- 

 tion it is found that exceedingly small specimens of the Medusa 

 make their appearance amongst the larger specimens. Mr. 

 .Sowerby had already determined this fact a fortnight ago, when 

 I first was introduced by him to the Medusa. I have now, 

 through his kindness, been able to examine several yonng phases 

 of Limnocodium, the discovery of which is entirely due to him. 



The youngest specimen which I have seen at present measured 

 only the one-thirtieth of an inch in diameter, and I have had 

 others under observation very little larger. The smaUest was 

 of a sub-spherical form without any aperture to the ectodermal 

 investment. Four minute tentacles were sprouting near one 

 pole of the spherical body, and between these rudiments of four 

 others were seen. Within — the subumbrellar musculature was 

 already developed and contracting at intervals. The four radial 

 canals were also present, and, what is more remarkable, the 

 sub-umbrellar cavity was already well marked, and within it the 

 manubrium with the oral aperture. Yet the margin of the 

 umbrella was stiU closed by a continuous ectodermal coat which, 

 when perforated, would, I conceive, become the velum. 



These minute embryos correspond veiy closely in appearance 

 with the embryos of the well-known typical Trachomedusan 

 Geryonia, as figured by Metschnikow in \he. Zeitsch. fiir zviss. 

 Zoologie, vol. .xxiv., Plate II., Figs. 12 and 15. 



They leave no possibihty of supposing that Limnocodium has, 

 like most Leptomedusa;, a hydroid trophosome. In respect of 

 its development as in other respects, Limnocodium is not more 

 closely allied to the Leptomedusae than to the Trachomedusce, 

 but is one of the Trachomedusfe. 



A remarkable fact which I am not able to explain is the 

 excessive rarity of females amongst the specimens of Limno- 

 codium taken from the tank in Regent's Park. All the speci- 

 mens which I have examined have been males. Females clearly 

 enough must be present, or have been present amongst the shoals 

 of males — whence the embryos discovered by Mr. Sowerby. 



It is a known fact among Trachyline Medusa; that in some 

 species males are excessively abundant, and even in some species 

 females have never been detected. Thus again Limnocodium 

 agrees with the Trachyline Medusae. 



One word more with regard to the name of the new Medusa. 

 Whilst I waive the right of priority for the generic term Craspe- 

 dacustes, and adopt Prof. Allman's term Limnocodium, I feel it 

 to be only right to maintain the association of Mr. Sowerby's 

 name with this discovery, which I had originaUy proposed, and 

 I shall accordingly henceforth speak of the Medusa as Limno- 

 codium Sowerliii, AUman and Lankester. 



E. Ray La.nkester 



Aqueous Vapour in Relation to Perpetual Snow 



Some twelve years ago I gave {Phil. Mag., March, 1867, 

 "Climate and Time," p. 548) what appears to be the true 

 explanation of that apparently paradoxical fact observed by Mr. 

 Glaisher, that the difference of reading between a thermometer 

 exposed to direct sunshine and one shaded diminishes, instead of 

 increases, as we ascend in the atmosphere. This led me to an 

 important conclusion in regard to the influence of aqueous 

 vapour on the melting-point of snow ; but recent objections to 

 some of my views convince me that I have not given to that 

 conclusion the prominence it deserves. I shall now state in a 

 few words the conclusion to which I refer. 



The reason why snow at great elevations does not melt but 

 remains permanent, is owing to the fact that the heat received 

 from the sun is thrown off into stellar space so rapidly by radia- 

 tion and reflection that the sun fails to raise the temperatm-e of 

 the snow to the melting-point ; the snow evaporates, but it does 

 not melt. The summits of the Himalayas, for example, must 

 receive more than ten times the amount of heat necessary to melt 

 all the snow that falls on them, notwithstanding which the snow 

 is not melted. And in spite of the strength of the sun and the 

 dryness of the air at those altitudes, evaporation is insufiicient to 

 remove the snow. At low elevations, where the snowfall is 

 probably greater and the amount of heat even less than at the 

 summits, the snow melts and disappears. This, I believe, 'we 

 must attribute to the influence of aqueous vapour. At high 

 elevations the air is dry and allows the heat radiated from the 

 snow to pass into space, but at low elevations a very considerable 

 amount of the heat radiated from the snow is absorbed in passing 

 through the atmosphere. A considerable portion of the heat 

 thus absorbed by the vapour is radiated back on the snow, but 

 the heat thus radiated, being of the same quality as that which 

 the snow itself radiates, is on this account absorbed by the snow. 

 Little or none of it is reflected like that received from the sun. 

 The consequence is that the heat thus absorbed accumulates in 

 the snow till melting takes place. Were the amount of aqueous 

 vapour possessed by the atmosphere sufficiently diminished, 

 perpetual snow would cover our globe down to the sea-shore. 

 It is tnie that the air is warmer at the lower level than at the 

 higher level, and by contact with the snow must tend to melt it 

 more at the former than at the latter position. But we must 

 remember that the air is warmer mamly in consequence of the 

 influence of aqueous vapour, and that were the quantity of 

 vapour reduced to the amount in question the difference of 

 temperature at the two positions would not be great. 



But it may be urged, as a further objection to the foregoing 

 conclusion, that as a matter of fact on great mountain chains 

 the snow-line reaches to a lower level on the side where the air 

 is moist than on the opposite side where it is dry and arid. As, 

 for example, on the southern side of the Himalayas and on the 

 eastern side of the Andes, where the snow-line descends some 

 2,000 or 3,000 feet below that of the opposite or dry side. But 

 this is owing to the fact that it is on the moist side that by far 

 the gi-eatest amount of snow is precipitated. The moist winds 

 of the south-west monsoon deposit their snow almost whoUy on 

 the southern side of the Himalayas, and the south-east trades 

 the snow on the east side of the Andes. Were the conditions in 

 every respect the same on both side-> of these mountain ranges, 

 with the exception only that the air on one side was perfectly 

 dry, allowing radiation from the snow to pass without interrup- 

 tion into stellar space, while on the other side the air was moist 

 and full of aqueous vapour absorbing the heat radiated from the 

 snow, the snow-line would in this case undoubtedly descend to a 

 lower level on the dry than on the moist side. No doubt more 

 snow would be evaporated ofl" the dry than off the moist side, 

 but melting would certainly take place at a greater elevation on 

 the moist than on the dry side, and this is what would mainly 

 determine the position of the snow-line. 



In like manner the dryness of the air will in a great measure 

 account for the present accumulation of snow and ice on Green- 

 land and on the Antarctic continent. I have shown on former 

 occasions that those regions are completely covered with perpetual 

 snow and ice, not because the quantity of snow falling on them is 

 great, but because the quantity melted is small. And the reason 

 why the snow does not melt is not because the amount of heat 

 received during the year is not equivalent to the work of melting 

 the ice, but, mainly because of the dryness of the air, the snow is 

 prevented from rising to the melting-point. 



There is little doubt but that the cold of the glacial epoch 

 would produce an analogous effect on temperate regions to that 



