The length of time during which wheel animals may be preserved in their quiescent state, and still retain the faculty of resurrection, has not hitherto been ascer tained. Leeuwenhoek kept them dry twenty-one months, yet they recovered on new humectation. Baker revived them after a longer period. Fontana preserved them two years ; and Spallanzani seems to infer, that he kept a quantity of sand containing these animals four years in a phial, and whenever a portion of it was moistened, they revived. " These facts," he continues, " must be understood under some limitation. Although the ani mals do revive repeatedly, and after remaining long dry, it is certain that the number always decreases in proportion to the time that the sand continues dry, and the times it has been wet for their resurrection. It is true, 1 have seen the eleventh resurrection. The first time, the wheel animals were very numerous; but their numbers continually diminished, and at length became very small. On still wetting and drying the sand, none revived the sixteenth time. Another portion of sand, numerously inhabited by wheel animals, was taken from roofs ; and I have preserved it dry in a box for three years, only moistening it every five or six months, for my observations. But the resurrections are always few er ; and it is no exaggeration to say, that now, at the end of the third year, a third do not revive." Humec tation with warm water produces the revival sooner than if cold is used.
There is one thing, however, indispensible to the re surrection of wheel animals, which is, the presence of sand. If it is wanting, they will never revive, though moisture be copiously supplied. Their members will partly swell indeed, from their dry and contracted state, but they will remain motionless. This is a circumstance which has been remarked by several naturalists ; and if a number of quiescent wheel animals are put into a watch-glass, in sonic parts of which there is sand, and in others none, it is only among the sand that they will be revived. If a considerable quantity is spread irregu larly over a surface, the resuscitant animals will be most numerous where it is most abundant. et Resurrection is also later among a small quantity than where there is much. If it succeeds here in four mi nutes, in the other case it will require nine or eleven. Further, it appears that privation of sand deprives them of their innate faculty of resurrection.
The heat of 113" kills the revived animalcula, whether it proceed from the solar rays or from fire. But in a state of desiccation, they can support between 155° and 158° without destruction. If the sand containing them is wet, when thus subjected to experiment, they do not revive after sustaining 131°. They also recover life, though the infusion which they inhabit has been expo sed to the cold of 11° below O. Strong and penetrating
odours, such as that of camphor, are fatal to them ; and the action of such seems to prevent the operation of the resurgent principle. Wheel animals revive sooner, and in greater numbers, in the open air than in vacuo ; and those that do not revive in vacuo, recover when brought into the open air. A vacuum preserved for a few days is fatal to them, as to all other animalcula.
The wheel animal is not the only one of the animal cular tribe that enjoys the wonderful prerogative of resurrection. Among the same sand which it inhabits, the abbe Spallanzani has discovered another minute animal, called the sloth, from its dull and sluggish na ture, which is endowed with a similar property. The colour of this microscopic being is a dark dingy yellow; and the surface of the body is granulated. It has six legs, each terminated by a hooked claw ; and the pos terior part is also terminated by four hooks. By these it attaches itself to the substances amidst which it dwells. The sloth never swims, its specific gravity being greater than that of water, whence it constantly crawls at the bottom of its infusion. But its motion is slow and languid ; and it is frequently seen lying su pine, and making great exertions to recover its proper position. This animal is three or four times larger than the wheel animal ; but its opacity prevents the internal structure from being discovered, unless in two spots, lighter coloured than the rest, which may be supposed' the stomach and oesophagus.
The sloth is infinitely more rare than the wheel ani mal. Spallanzani remarks, that it is hardly one sixth as numerous ; and in a long series of experiments made in Scotland, between the years 1800 and 180-1, it occur red only in two infusions. It is true, that it was not industriously sought for ; but there were hundreds of wheel animals.
The phenomena attending the death and resurrection of the sloth, are precisely the same as those of the wheel animal. Motion gradually ceases as the water evapo rates; the tentacula are contracted, and drawn within the body, which diminishes in size, assumes a globular figure, and becomes clry and hard. But all the parts 'expand on the allusion of water. The sloth revives only a limited number of times ; and although sand is necessary for its resurrection, it does not appear so es sential as for the revival of the wheel animal. The same degrees of heat are fatal to it ; and it is equally capable of resisting cold. Its mode of propagation is unknown. Perhaps the sloth does not •strictly belong to the animalcula infusoria ; but we are not aware that it has been received into any other order.