Jtine 17, 1880] 



NA rURE 



147 



thesis, these in their turn would produce a secular 

 magnetic change. 



(y) Regarding Earth Currents and Auroras. — I have 

 for some considerable time looked on the earth as a 

 Ruhmkorff's coil with a magnetic nucleus. Above this 

 nucleus we may suppose that we have the primary rocks, 

 which are non-conductors, while above these we have 

 the moist or comparatively moist surface of the earth, 

 which is a conductor. Above this, again, we have the 

 lower strata of the atmosphere, which are non-conductors, 

 while above this we have the upper strata, which are 

 conductors. 



Now suppose that a small but abrupt change of the 

 earth's m.agnetism takes place, no matter how. We need 

 not enter into the causes of such. 



We have thus t'.vo secondary coils, if I may use the 

 expression : (i) the moist surface of the earth, (2) the 

 upper regions of the atmosphere ; and both of these will 

 be animated with secondary currents, on account of the 

 abrupt change of the earth's magnetism. These secondary 

 currents will be in one direction for a magnetic change of 

 one kind, and in the opposite direction for a magnetic 

 change of the opposite kind. 



Now whenever there are magnetic storms, that is to 

 say, when there are small but abrupt changes of the 

 earth's magnetism, it is well known from the Greenwich 

 records that w-e have violent earth currents, which are 

 alternately positive and negative, and that w-e have also 

 auroral displays in the upper regions of the earth's atmo- 

 sphere. We cannot examine the auroral displays as we 

 can the earth currents. But with regard to earth currents 

 I would remark that the /oriii of the phenomena they 

 display is entirely against the supposition that such 

 currents are the main cause of the changes in terrestrial 

 magnetism, and in favour of that which maintains that 

 they are secondary currents induced by magnetic changes. 



In conclusion I would guard against its being supposed 

 that all luminous appearances in the atmosphere are due 

 to the same cause. I only hold that certain appearances 

 which occur at times of magnetic perturbation and 

 simultaneously throughout a large portion of the earth 

 have the origin now mentioned. B. Stewart 



ON A NEW JELLY-FISH OF THE ORDER 

 TRACHOMEDUS.E, LIVING IN FRESH 

 WATER 



(~\^ Thursday last, June 10, Mr. Sowerby, the secretary 

 ^-^ of the Botanical Society of London, observed in the 

 tank in the water-lily house in Regent's Park a peculiar 

 organism, of which he was kind enough to place a large 

 number at my disposal on the following Monday. 



The organism proves to be an adult medusa belonging 

 to the order Trachomedusa; and the family Petasidze of 

 Haeckel's system ("System der Medusen," erster Theil). 

 It comes nearest among described genera to Fritz Miiller's 

 imperfectly known Aglaurapsis from the coast of Brazil. 



The most obviously interesting matter about the form 

 under notice is that it occurs in great abundance in 

 perfectly fresh water at a temperature of 90^ Fahr. 



Hitheito no medusa of any order has been detected in 

 fresh water — e.xcept perhaps some stray estuarine forms 

 (? Crambessa). 



It is exceedingly difficult to trace the introduction of 

 this animal into the tank in the Regent's Park, since no 

 plants have been recently (within twelve months) added 

 to the lily-house, and the water is run off every year. 

 Probably a few specimens were last year or the year 

 before present in the tank, and have only this year 

 multiplied in sufficient abundance to attract attention. 

 Clearly this medusa is a tropical species, since it flourishes 

 in water of the high temperature of 90° Fahr. 



Mr. Sowerby has observed the medusa feeding on 

 Daphnia, which abcunds in the water with it. 



The present form will have to be placed in a new genus, 

 for which I propose the name Craspedacusta, in allusion 

 to the relation of its otocysts to its velum. 



It is one of the sub-class Hydromeduste or Medusa; 

 craspedota;, and presents the common characters of the 

 order Trachomedusffi (as distinguished from the Narco- 

 medusa;) in having its genital sacs or gonads placed in the 

 course of the radial canals. It agrees with allTracholina; 

 (Trachomedusre and Narcomcdusre) in having endodermal 

 otocysts, and it further exhibits the solid tentacles with 

 cartilaginoid axis, the centripetal travelling of the ten- 

 tacles, the tentacle rivets (Mantelspangen), the thickened 

 marginal ring to the disk (Xessel ring) observed in many 

 Tracholinse. 



Amongst Trachomedusas, Craspedacusta finds its place 

 in the Petasidse, which are characterised as " Tracho- 

 medusjc with fotir radial canals, in the course of which 

 the four gonads lie, with a long tubular stomach and no 

 stomach-stalk." 



Amongst Petasidas it is remarkable for the great number 

 of its tentacles, which arc all solid ; and for its very 

 numerous otocysts. Further, it is remarkable among all 

 Hydromedusa; (velate medusae, that is, exclusive of 

 Charybda:a) for the fact that centrifugal radiating canals 

 pass from the otocysts into the velum, u>he>-e they end 

 ccecally. 



The genus may be characterised as follows : — 



Mouth quadrifid, with four per-radial lobes. 



Stomach long, quadrangular, and tubular, projecting 

 a good deal below the disk. 



Disk, saucer-shaped, that is, flattened. 



Radiating Canals 4, terminating blindly at the 

 margin of the disk. 



Gonads 4, in the form of 4 oval sacs, depending into the 

 cavity of the subumbrella from the four radiating canals. 



Marginal or Ring Canal obliterated (or if present 

 of very minute size). 



Centripetal Canals (such as those of Olindias, 

 Geryonia, &c.) absent. 



Tentacles solid ; in three sets, which are placed in 

 three superimposed horizons :^ 



1. A set nearest the aboral pole, of 4 large per-radial 



tentacles. These are the primary tentacles. 



2. A second tier of (in large specimens) 28 medium- 



sized tentacles placed between these in four groups 

 of seven. These are the secojidary tentacles. 



3. A third tier of (in large specimens) 192 small ten- 



tacles placed in groups of six between adjacent 

 secondary tentacles. These are the tertiary 

 tentacles. 



Tentacle-Rivets (Mantel-spangen) connecting the 

 roots of the tentacles with the marginal ring (Nessel-ring) 

 are connected with all the tentacles of each of the three 

 horizons. 



Otoliths placed along the line of insertion ot the 

 velum— about eighty in number (fewer in small specimens). 

 From sixteen to twenty are placed between successive per- 

 radial tentacles arranged in groups of two or three between 

 the successive secondary tentacles. 



Velar Centrifugal Canals (which are really the 

 elongated otocysts) are peculiar to this genus, passing 

 from the otoliths (one inclosing each otolith) into the 

 velum, and there ending blindly. They appear to corre- 

 spond in character to the centripetal canals found in other 

 Trachomedusre in the disk. 



Ocelli are absent. 



[The presence of velar otocystic canals constitute the 

 chief peculiarity of the genus Craspedacusta, and may 

 necessitate the formation of a distinct family or sub-order 

 for its reception. The minute structure of the otoliths 

 and canal-like otocysts 1 am now engaged in investigating.] 



The above characters are derived from the examination 

 oi adult male specimens, which were freely discharging 

 ripe, actively motile spermatozoa. 



