March 6, 1884] 



NA TURE 



431 



and Distomidae do not correspond exactly to any of 

 Giard's families, but the former is Milne-Edwards' " Poly- 

 cliniens " without change. A new family, the Chondro- 

 stachyidse, has been formed for the reception of Mac- 

 donald's C/ioiidrostac/iys and von Dra^che's Oxycorynia, 

 remarkable forms in which the Ascidiozooids are placed 

 upon a common peduncle penetrated by large canals. I 

 am inclined to admit the necessity for this new family, and 

 several undescribed and interesting forms obtained during 

 the CJuilleiigcr Expedition will, 1 hope, take up a position 

 within its bounds. The two remaining families of von 

 Drasche's system, the ClavelinidK and the Perophoridae, 

 I would still maintain arc more closely allied to the simple 

 than to the compound Ascidians. They correspond to 

 Family IV. Clavelinida; of my arrangement of the Ascidiae 

 Simplices. 



Dr. von Drasche does not define the Synascidiae, and 

 from one or two passages in his work it seems probable 

 that he is in very much the position in which I now find 

 myself, viz. unable to find any character or combination 

 of characters which will serve to distinguish simple from 

 compound Ascidians. Reproduction by gemmation and 

 the formation of colonies in the latter group will not hold, 

 since it is possible to pass from Ciona — a typical simple 

 .Ascidian — to Distoma and the very heart of the compound 

 Ascidians through the following series of forms, which 

 shows a perfect gradation of these characters : — Cioini, 

 R/iopahra, Ecteiiiascidia, Clai'cli/ia, Diacomi, Cliondro- 

 stacliys, Oxycorynia, DistovuT. The formation of common 

 cloacal cavities, canals, and apertures cannot be con- 

 sidered as a diagnostic feature of the compound Ascidians. 

 Although Giard has demonstrated tlieir presence in some 

 genera in which they were previously unknown, yet there 

 are some forms considered by all authorities as Syn- 

 ascidiae, such as C/iondios/achys, Diazona, Distoma, and 

 others, in which the atrial apertures of the Ascidiozooids 

 open independently on the surface of the colony, and no 

 common cloaca is formed. 



Lastly, we come to characters taken from the condition 

 of the test, but these break down like the others. In the 

 first place, in passing along the series of forms mentioned 

 above as connecting Ciona and Distoma, we encounter all 

 stages between a distinct test or tunic for each individual 

 and a common mass in which a number of Ascidiozooids 

 arc embedded. And, secondly, the remarkable group 

 " Polystyete," briefly characterised by Giard in 1S74 

 presents many of the characters of highly differentiated 

 simple Ascidians (the Cynthiidze), along with the sup- 

 posed Sjnascidian feature of a colony composed of many 

 -Ascidiozooids completely buried in a common test. 



In the Challenger collection there is an interesting series 

 of Polystyelae — all from southern seas — in which it is 

 possible, I believe, to trace a passage from such aggre- 

 gated Stselinae as Polycarpa to the Botryllidse. If this 

 passage indicated genetic affinity between these two very 

 distinct groups, which I greatly doubt, it would be impos- 

 sible to escape from the conclusion that the Ascidife 

 Simplices and the Ascidise Compositae have two points of 

 connection, almost at the extreme ends of the two series. 

 I think I am justified in believing that probablv both 

 groups were derived from a form not unlike Eitei)iascidia 

 or Clavelina. From this common ancestor the simple 

 .Ascidians diverged through the Ascidiidse to the Cynthiidae 

 (including Polycarpa) and the Molgulida;, while the com- 

 pound Ascidians diverged through Diazona and the 

 Cbondrostachyidae to the Polyclinidas, Didemnidae, and 

 Botryllida;. Hence it seems much more probable that 

 the Polystyela: have acquired independently certain cha- 

 racters of Polycarpa or oi Botrylliis (I have not yet been 

 able to determine to which of the two they are really most 

 closely related) than that there is any direct affinity 

 between such highly differentiated groups as the Cynthiidte 

 and the Botryllidte. This, however, does not affect the 

 practical difficulty that the Polystyelae completely bridge 



across the gap between simple and compound Ascidians 

 as distinguished by the nature of the test or tunic, and 

 consequently it is extremely difficult to separate tl em 

 from either of these two great series. 



Thus all the diagnostic features usually employed fail 

 utterly, and we find ourselves unable to discover a sing'e 

 character or combination of characters which will serve 

 to distinguish the AscidiK Simplices from the AscidiK 

 Composita?. W. A. Herdman 



./ METEOROLOGICAL LABORATORY 



'X'O the last issue of Science et Nature M. L. Mangin 

 -'• contributes an interesting account of the chemical 

 laboratory recently installed on the Pic du Midi, Pyrenees, 

 at an altitude of nearly 9500 feet above the sea. As 

 shown in our first illustration, the laboratory stands 

 between the dwelling-house and the Observatory, of which 

 it forms a dependency, under the direction of MM. 

 Miintz and Aubin. In the second illustration a fuller 

 view is given of the building, which faces southwards, 

 and the slated roof of which is so constructed as to con- 

 stitute a sort of pluvioineter registering the annual rain- 

 fall, and retaining sufficient for chemical analysis. This 

 unique establishment, which promises to render great 

 services both to meteorology and to the economic 

 industries, is at present chiefly occupied with the con- 

 stituent elements of the terrestrial atmosphere, especially 

 in connection with vegetable life. The student of che- 

 mistry need scarcely be reminded that, besides oxygen 

 and nitrogen, the air contains in smaller proportions 

 carbonic acid, ammonia, and certain nitric compounds 

 playing an important part in the nutrition of plants, and 

 supplying them with nearly all the nitrogen and carbon 

 that enter into the composition of their tissues. During 

 the summer months of the >ears iSSi-S.', MM. Miintz 

 and Aubin were mainly engaged with the quantitative 

 analysis of these substances, under conditions peculiarly 

 favourable for the prosecution of such investigation^. 

 The results so far obtained may here be briefly resumed. 



Carbonic Acid. — The proportion of this element found 

 in the air at different altitudes is still a subject of discus- 

 sion amongst analytical chemists. But de Saussure's 

 average of from '0004. to '0006 has been shown to be con- 

 siderably too high by various observations taken of late 

 years at different stations on the globe. These observa- 

 tions are now fully confirmed by the researches on the 

 Pic du Midi, which reduce the average to 2-86 ten- 

 thousandths. 



Another important conclusion is that the carbonic acid 

 does not perceptibly vary with the ahitude, as had hitherto 

 been supposed. Thus the proportion is found to be much 

 the same at Vincennes near Paris, Luz (740 m.), Pierrefitte 

 (500 m.), and Pic du Midi (2900 m.). On the other hand, the 

 quantity varies slightly in the same locality, beingsomewhat 

 greater at night and in moist weather than during the day 

 and in dry weather. The subjoined table shows the 

 average quantity of carbonic acid present in the atmo- 

 sf here during the day and at night at various meteoro- 

 logical stations in different parts of the world : — 



Night Day 



Vincennes 298 ... 2'S4 



Fie dii Midi 2-90 ... 286 



Hayti 2-92 ... 270 



Florida 2-94 ... 2^89 



Martinique 2 85 ... 27? 



Mexico 286 ... 266 



Patagonia 2*67 ... 2'65 



Chili 2-82 ... 2-66 



Ammonia. — Although the presence of ammonia in the 

 air has long been known, Schlosing was the first to show 

 that for this substance, as well as for carbonic acid, the 

 sea is the great reservoir whence the atmosphere receives 

 ts supplies. I'ut no light hadhitheito been thrown upon 



