June 28, 1888] 



NATURE 



197 



this year ; but I first saw them on June 12, and again on the 

 14th ; and I think I saw them on June 13 and 17, but was not 

 sure. Previous to that, on May 15 and 16, the green sky, when 

 the sun had set, was of unusual brightness, showing, as I 

 thought, a tendency to the formation of these clouds. Each 

 summer they appear to be growing fainter since they were first 

 generally noticed in 1885. 



This year's observations were made in Cornwall, with the 

 exception of last night's, which was at Sunderland. 



Sunderland, June 26. T. W. Backhouse. 



Earth Pillars in Miniature. 



I HAVE taken two photographs of an interesting specimen I 

 obtained from the cliffs here. The stone is composed of very 

 fragile sand-rock containing fragments of flint. A large mass 

 of this became detached from the higher part of the cliff, and 

 some of the pieces chanced to fall on a ledge upon which dry 

 sand was constantly pouring in windy weather. The action of 

 this falling sand wore away all parts of the surface of the stone 

 save those protected by the small embedded fragments of flint, 

 and hence the formation of these miniature pillars. 



Owing to the extreme incoherency of the substance, I un- 

 fortunately lost one of the most perfect pillars before the 

 photograph was taken. 



I conclude that the formation of these pillars was the work of 

 a very few days — perhaps hours. On visiting the spot a few 

 days later, all traces of sand-action had been obliterated by 

 rain. An analogous case was that described by Mr. Blake 

 ("Geol. Miscell. Tracts," 10) as occurring in the Pass of San 

 Bernardino, California ; the surface of the granite had been 

 worn by blown sand, but the garnets therein stood out in relief 

 upon long pedicles of feldspar, as a proof of their superior 

 hardness. Cecil Carus-Wilson. 



Bournemouth, June 23. 



Egg-masses on Hydrobia ulvce. 



Can any of your readers give me information in regard to the 

 eggs of the Gastropod Hydrobia ulvce ? 



At a recent excursion of the Biological Society to Hilbre 

 Island, while crossing the great stretch of wet sand which lies in 

 the estuary of the Dee, it was noticed that the surface was 

 covered in some places with vast numbers of Hydrobia. Some 

 of these were brought back to the laboratory in their wet sand ; 

 and, on being put in a dish of sea-water, the mollusks were 

 found next day to have crawled out of the sand, and I then 

 noticed that nearly every specimen had several little rounded 

 excrescences scattered over the surface of its shell. On examin- 

 ing these, it was found that each was a little mass of small 

 sand grains, in the centre of which was a clear jelly containing 

 several segmenting ova or young embryos. They were undoubt- 

 edly molluscan eggs, as I kept them alive until one or two had 

 reached a veliger stage ; but did they belong to the Hydrobia 

 or to some other mollusk ? No other mollusk was, however, 

 noticed in any abundance in the neighbourhood. Has, then, 

 the Hydrobia acquired the habit of laying its eggs upon its 

 neighbours' shells, as being the only comparatively stable objects 

 to be found in the fine shifting sands around it ? Possibly the 

 method of oviposition of Hydrobia is already known, but 1 have 

 not come across any reference to it. W. A. Herdman. 



Zoological Laboratory, University College, Liverpool, 

 June 23. 



Interpretation of the Differential Equation to a Conic. 



May I ask, with reference to Mr. Asutosh Mukhopadhyay's 

 geometrical interpretation of the above in Nature of the 21st 

 inst., how to draw a curve at every point of which the radius of 

 curvature vanishes, or the curvature is infinite? 



Is it not evident that the osculating conic of a conic is the 

 conic itself, and the " aberrancy curve " therefore a point, the 

 centre of the conic ? 



The "sought found," then, is the fact that a conic is a conic ! 



June 24. R. B. H. 



The Nephridia of Earthworms. 



The last number of the Quarterly Journal of Microscopical 

 Science has just come into my hands, containing a paper, by Mr. 



Beddard, on the nephridia of certain earthworms. In Novem- 

 ber of last year I read a paper, before the Royal Society of 

 Victoria, on the anatomy of the large Gippsland earthworm, 

 Megascolides australis. This, which reaches the length of 6 to 

 8 feet, is, I believe, the largest recorded earthworm, and its 

 nephridial system is of great interest, corresponding closely in 

 many points to that described by Mr. Beddard, in the above 

 paper, as present in Acanthodrilus multiporus and Perichata 

 aspergillum. My drawings have been for some time in the 

 lithographers' hands, but as it will still be one or two months 

 before the full paper is published, I should be glad to draw 

 attention to .the, in some ways, still more interesting features of 

 the nephridial system in Megascolides australis. The nephridia 

 are very evident, and can be divided clearly into two sets. 



(1) A great number of small vascular-looking little tufts lining 

 the body- wall, save in the mid-dorsal and ventral lines, espe- 

 cially abundant in the segments containing the reproductive 

 organs (segments 11-19). They have no internal opening. 



(2) A series of much larger nephridia, one pair of which only 

 is present in each of the segments in the middle and posterior 

 regions of the body — that is, from about segment 120 to segment 

 500, or whatever may be the number of the last segment, which 

 varies according to the worm's size. They are placed in the 

 anterior part of each segment, whilst the smaller nephridia form 

 a ring round the body-wall posteriorly. Each one has the usual 

 ciliated funnel opening through the septum into the segment in 

 front. 



Throughout the body, where the smaller nephridia occur, 

 there is a network of intra-cellular ducts lying immediately 

 beneath the peritoneal epithelium in connection with the 

 nephridia, and giving off an irregularly arranged series of 

 branched ducts opening externally. Ventrally, also, there 

 appears to be on either side, in the middle and posterior por- 

 tions of the body, a longitudinal duct running from segment to 

 segment within the most ventral pair of setae : into this duct 

 open, first, the larger nephridia, and, secondly, the most vent- 

 rally placed small nephridia of the same segment ; the latter, 

 again, are united with the network of ducts connected with the 

 ring of smaller nephridia. 



In the case of the latter there appear to be two somewhat 

 differently formed sets of external openings. All over the body, 

 except in the clitellar region, where there is a great glandular 

 development in the body-wall, the duct leading to the exterior 

 is intercellular, small, and composed of minute cubical cells ; in 

 the clitellar region, on the other hand, the duct, though similarly 

 intercellular, is much swollen out, slightly coiled, and always 

 provided with a distinct coiled blood-vessel running by its side : 

 its lining cells form a flattened epithelium. 



The external opening itself is formed of cells of the epidermis, 

 so modified as to present very much the external appearance of 

 a taste-bulb — that is, they form a sphere with the cells thicker in 

 their middle parts, and the two ends attached to the poles of the 

 sphere, the duct passing right up through the centre. This 

 structure of the external opening is common to all the ducts in 

 the body, but is more clearly made out in the case of those 

 referred to. 



The large size and ciliated funnels of the paired nephridia 

 distinguish these clearly from the more numerous smaller ones, 

 which are devoid of internal openings, and are without a doubt 

 homologous with those of Acanthodrilus and Perichceta. At the 

 same time it is important to note that histologically the network 

 of ducts and the longitudinal duct, which are intimately con- 

 nected with each other, are precisely similar in structure, and, 

 a priori, might be expected to have a similar origin, i.e. to be 

 derived from the same germinal layer. 



Leaving out of consideration at present the question dealt 

 with by Mr. Beddard and others as to the homology of the larval 

 nephridia of Choctopods, and assuming the existence of a genetic 

 relationship between the adult nephridial system of Platy helminths 

 and Chsetopods, the following questions suggest themselves with 

 regard to the various nephridial structures present in different 

 forms : — 



(1) Are the longitudinal ducts in Lanice, the embryo of 

 Lumbricus and Megascolides, homologous with each other? 

 Before this can be determined the development of each must be 

 known. 



(2) Granted, of which there can be little doubt, that the 

 smaller nephridia of Megascolides are homologous with the 

 nephridia of Perichata and Acanthodrilus, are not the large 

 nephridia of the former, which are completely wanting in both 



