SCIENCE. 



45 



some distance above it, while in Limnocodium the ridges 

 are direct continuations of the tentacles whose structure 

 they retain. They become narrower as they approach the 

 margin. 



The number of the tentacles is very large in adut speci- 

 mens. The four tentacles which correspond to the direc- 

 tions of the four radial canals, or the perradial tentacles, are 

 the longest and thickest. The quadrant which intervenes 

 between every two of these carries, at nearly the same 

 height above the margin, about thirteen shorter and thinner 

 tentacles, while between every two of these three to five 

 much smaller tentacles are given off from points nearer to 

 the margin, and at two or three levels, but without any 

 absolute regularity ; indeed, in the older examples all regu- 

 larity, except in the primary or perradial tentacles, seems 

 lost, and the law of their sequence ceases to be apparent. 



I could find no indication of a cavity in the tentacles ; but 

 they do not present the peculiar cylindrical chord-like 

 endodermal axis formed by a series of large, clear, thick- 

 walled cells which is so characteristic of the solid tentacles 

 in the Tiachomedusae and Narcomedusae. From the solid 

 tentacles of these orders they differ also in their great ex- 

 tensibility, the four perradial tentacles admitting of exten- 

 sion in the form of long, greatly-attenuated filaments to 

 many times the height of the vertical axis of the umbrella, 

 even when this height is at its maximum ; and being again 

 capable of assuming by contraction the form of short thick 

 clubs. Indeed, instead of presenting the comparatively 

 rigid and imperfectly contractile character which prevails 

 among the Trachomedusae and the Narcomedusae, they 

 possess as great a power of extension and contraction as 

 may be found in the tentacles of many Leptomedusae (Thau- 

 mantidae, &c.) These four perradiate tentacles contract in- 

 dependently of the others, and seem to form a different 

 system. All the tentacles are armed along their length with 

 minute thread cells, which are set in close, somewhat spir- 

 ally arranged warts. 



The lithocysts or marginal vesicles are, in adult speci- 

 mens, about 128 in number. They are situated near the 

 umbrellar margin of the velum, between the bases of the 

 tentacles, and are grouped somewhat irregularly, so that 

 their number has no close relation with that of the tentacles. 

 They consist of a highly refringent spherical body, on which 

 may be usually seen one or more small nucleus-like cor- 

 puscles, the whole surrounded by a delicate transparent 

 and structureless capsule. This capsule is very remarkable, 

 for instead of presenting the usual spherical form, it is of 

 enlongated piriform shape. In its larger end is lodged the 

 spherical refringent body, and it thence becomes attenuated, 

 forming a long tubular tail-like extension which is continued 

 into the velum, in which it runs transversely towards its 

 free margin, and there, after usually becoming more or less 

 convoluted, terminates in a blind extremity. 



The marginal nerve-ring can be traced running round 

 the whole margin of the umbrella, and in close relation 

 with the otolitic cells. Ocelli are not present. 



The generative sacs are borne on the radiating canals, 

 into which they open at a short distance beyond the exit 

 of these from the base of the manubrium. They are of an 

 oval form, and from their point of attachment to the radial 

 canal hang down free into the cavity of the umbrella. 

 Some of the specimens examined contained nearly mature 

 ova, which, under compression, were forced from the sac 

 through the radial canal into the cavity of the stomach. 



While some of the characters described above point to 

 an affinity with both the Trachomedusae and Narcomedusae, 

 this affinity ceases to show itself in the very important 

 morphological element afforded by the marginal bodies. 

 In both Trachomedusae and Narcomedusae the marginal 

 bodies belong to the tentacular system ; they are metamor- 

 phosed tentacles, and their otolite cells are endodermal, 

 while in the Leptomedusae, the only other order of craspe- 

 dotal Medusae in which marginal vesicles occur, these 

 bodies are genetically derived from the velum. Now in 

 Limnocodium the marginal vesicles seem to be as truly 

 velar as in the Leptomedusae. They occur on the lower 

 or abumbral side of the velum, close to its insertion into 

 the umbrella, and the tubular extension of their capsule 

 runs along this side to the free margin of the velum, while 



the delicate epithelium of the abumbral side passes over 

 them as in the Leptomedusae. It is true that this point 

 cannot be regarded as settled until an opportunity of trac- 

 ing the development is afforded ; but in very young speci- 

 mens which I examined I found nothing opposed to the 

 view that the marginal vesicles were derived, like those of 

 the Leptomedusae, from the velum. 



Important points still remain to be cleared up regarding 

 the development of Limnocodium and the determination of 

 the question whether the Medusa be derived from the egg 

 directly or only through the intervention of a hydranlid 

 trophosome. I have arranged, with Mr. Sowerby, some 

 methods of observation by which I hope to obtain data for 

 determination of these points. 



If this be the case Limnocodium will hold a position in- 

 termediate between the Leptomedusae and the Trachome- 

 dusae ; but as the greatest systematic importance must be 

 attached to the structure and origin of the marginal vesicles, 

 its affinity with the Leptomedusae must be regarded as the 

 closer of the two. Geo. J. Allman. 



THE ELECTRIC LAMPS OF M. TCHJKOLEFF. 



M. Tchikoleff, the head of the electric lighting depart- 

 ment of the Russian artillery, has addressed to La Lumiere 

 Electrique a communication, of which the following is a 

 translation, in which he claims that the application of de- 

 rived currents which has been successfully adopted with 

 the lamps of MM. Lontin and Siemens, was employed by 

 him as far back as the year 1871. 



" Having experimented for a lengthened period with the 

 Foucault and Serrin regulator lamps, which were con- 

 sidered to be the best at the period when I took up the 

 question, I was able to observe in them the following 

 defects : 



1. Several lamps, arranged in series or in multiple arc in 

 a circuit, would not continue to work. 



2. These lamps could be worked only by very powerful 

 currents, whereas with a lamp regulated by hand the 

 voltaic arc could be obtained with weaker currents, giving 

 of course a less intense light. 



3. They worked with regularity only when the current 

 was constant, or varied within very restricted limits. 



I traced the cause of these defects to the fact that the 

 working of the regulating mechanism was based upon a 

 kind of equilibrium between the attractive force of an 

 electro-magnet and the counteracting force of a spring. 

 Such a system does not regulate the distance between the 

 charcoal points, but only the general force of the current in 

 the circuit. Now under these circumstances it is possible 

 that, when two or more lamps are placed in series in a 

 circuit, one of them may have its carbons in contact, whilst 

 the carbons of the other lamp or lamps are at a greater or 

 less distance apart, without the equilibrium Detween the 

 electro-magnets and the counteracting springs being dis- 

 turbed. 



Now it was to obviate this defect that I endeavored to 

 devise an arrangement which, whilst allowing each lamp 

 placed in a circuit to be independent of the general inten- 

 sity of the current and its variations, would enable it to 

 maintain constant the resistance of its own voltaic arc, and 

 this arrangement appeared to me obtainable by applying to 

 the regulator lamps the principle of the differential action of 

 derived currents. 



It was in 1869 that I made the first experiment on the 

 foregoing arrangement with a regulator lamp of M. Foucault, 

 the counteracting spring of which I replaced by a supple- 

 mentary electro-magnet traversed by a very weak deriva- 

 tion of the current, parallel to the voltaic arc. This electro- 

 magnet was wound with a wire of high resistance, and the 

 current producing the voltaic arc passed through the other 

 electro-magnet. The armatures of these electro-magnets 

 were placed at the two extremities of a rocking-lever, 

 carrying at its centre of oscillation an arm which controlled 

 the mechanism for increasing or diminishing the dis- 

 tance between the carbons ; and the rocking-lever was in 

 equilibrium when the voltaic arc possessed its normal re- 

 sistance. 



