Jiuu 24, iSSoJ 



NA TURE 



179 



length with minute thread cells, which are set in closei 

 somewhat spirally-arranged warts. 



The lithocysts or marginal vesicles are, in adult 

 specimens, about 12S in number. They are situated 

 near the umbrellar margin of the velum, between the 

 bases of the tentacles, and are grouped somewhat irregu- 

 larly, 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 corpuscles, the whole surrounded 

 by a delicate transparent and structureless capsule. This 

 capsule is ver)- remarkable, for instead of presenting the 

 usual spherical form, it is of an elongated piriform shape. 

 In its larger end is lodged the spherical refringent body, 

 and it thence becomes attenuated, forming a long tubular 

 tail-Uke 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 um- 

 brella. 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 Trachomedusa and Narcome- 

 dusre, this affinity ceases to show itself in the very im- 

 portant morphological element afforded by the marginal 

 bodies. In both Trachomedusa: and Narcomedusas the 

 marginal bodies belong to the tentacular system; they are 

 metamorphosed tentacles, and their otolite cells are endo- 

 dermal, while in the Leptomedusre, the only other order 

 of craspedotal Medusa? 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 Leptomedusa;. It is true that this 

 point cannot be regarded as settled until an opportunity 

 of tracing the development is aflorded ; but in very young 

 specimens 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 Limnccodium and the determination 

 of the question whether the Medusa be derived from the 

 egg directly or only through the intervention of a hy- 

 dranlid trophofome. 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 

 intermediate between the Leptomedusse and the Tracho- 

 medusae ; but as the greatest systematic importance must 

 be attached to the structure and origin of the marginal 

 vesicles, its affinity with the Leptomedusse must be 

 regarded as the closer of the two. Geo. J. Allman 



Physiology of tlie Freshwater Medusa 

 The structure of this remarkable animal has already 

 been investigated and described by Professors .Allman 

 and Lankester, with the result of showing that, although 

 constituting a new genus, it is in all respects a true Medusa. 

 After the publication of their papers I began to work out 



the physiology of the new form, and the following are 

 the results which so far I have obtained. 



The natural movements of the IMedusa precisely 

 resemble those of its marine congeners. More particu- 

 larly, these movements resemble those of the marine 

 species which do not swim continuously, but indulge in 

 frequent pauses. In water at the temperature of that in 

 the Victoria Lily-house fSj" F.) the pauses are frequent, 

 and the rate of the rhythm irregular — suddenly quickening 

 and suddenly slowing even during the same bout, which 

 has the effect of giving an almost intelligent appearance 

 to the movements. This is especially the case with young 

 specimens. In colder water (65° to 75") the movements 

 are more regular and sustained ; so that, guided by the 

 analogy furnished by my experiments on the marine 

 forms, I infer that the temperature of the natural habitat 

 of this Medusa cannot be so high as that of the water in 

 the Victoria Lily-house. In water at that temperature 

 the rate of the rhythm is enormously high, sometimes 

 rising to three pulsations per second. But by progres- 

 sively cooling the water, this rate may be progressively 

 lowered, just as in the case of the marine species ; and in 

 water at 65^ the maximum rate that I have observed is 

 eighty pulsations per minute. As the temperature at 

 which the greatest activity is displayed by the fresh- 

 water species is a temperature so high as to be fatal to 

 all the marine species which I have observed, the effects 

 of cooling are of course only parallel in the two cases 

 when the eftects of a series of higher temperatures in 

 the one case are compared with those of a series of 

 lower temperatures in the other. Similarly, while a 

 temperature of 70' is fatal to all the species of marine 

 Medusffi which I have examined, it is only a temperature 

 of 100° that is fatal to the freshwater species. Lastly, 

 while the marine species will endure any degree of cold 

 without loss of life, such is not the case with the fresli- 

 w-ater species. Marine Medusa:, after having been frozen 

 solid, will, when gradually thawed out, again resume their 

 swimming movements ; but this freshwater IMedusa is 

 completely destroyed by freezing. Upon being thawed 

 out, the animal is seen to have shrunk into a tiny ball, and 

 it never again recovers either its life or its shape. 



The animal seeks the sunlight. If one end of the tank 

 is shaded, all the Medusa; congregate at the end which 

 remains unshaded. Moreover, during the daytime they 

 swim about at the surface of the water ; but when the sun 

 goes down they subside, and can no longer be seen. In 

 all these habits they resemble many of the sea-water 

 species. They are themselves non-luminous. 



I have tried on about a dozen specimens the effect of 

 excising the margin of the nectocalyx. In the case of all 

 the specimens thus operated upon, the result was the 

 same, and corresponded precisely with that which I have 

 obtained in the case of marine species. That is to say, 

 the operation produces immediate, total, and permanent 

 paralysis of the nectocalyx, while the severed margin con- 

 tinues to pulsate for two or three days. The excitability 

 of a nectocalyx thus mutilated persists for a day or two, 

 and then gradually dies out — thus also resemlaling the 

 case of the marine naked-eyed Medusae. More particu- 

 larly, this excitability resembles that of those marine 

 species which sometimes respond to a single stimulation 

 with two or three successive contractions. 



A point of specially physiological interest may be here- 

 noticed. In its unmutilated state the freshwater Medusa 

 exhibits the power of localising with its manubrium a 

 seat of stimulation situated in the bell. That is to say, 

 when a part of the bell is nipped with the forceps, or 

 otherwise irritated, the free end of the manubrium is 

 moved over and applied to the part irritated. So far, the 

 movement of localisation is precisely similar to that which 

 I have previously described as occurring in Tiaropsis 

 i>idicans{Phil. Trans., \o\. clxvii.). But further than this, 

 I find a curious difference. For while in T. iiidicans 



