OF MOISTURE AS A CONDITION OF VITAL ACTIVITY. 103 



dria, which at certain seasons become so dry, that their beds are burnt 

 as hard as bricks by the action of the sun, so as to be fit for use as car- 

 riage roads ; yet the plants do not thereby lose their vitality; for when 

 the water is again admitted, they resume their growth with redoubled 

 vigour. 



159. Among the lower Animals, we find several of considerable com- 

 plexity of structure, which are able to sustain the most complete desic- 

 cation. This is most remarkably the case in the common Wheel-Ani- 

 malcule; which may be reduced to a state of most complete dry ness, 

 and kept in this condition for any length of time, and which will yet 

 revive immediately on being moistened. The same individuals may be 

 treated in this manner, over and over again. Experiments have been 

 carried still further with the allied tribe of Tardigrades ; individuals of 

 which have been kept in a vacuum for thirty days, with sulphuric acid 

 and Chloride of Calcium (thus suffering the most complete desiccation 

 the Chemist can effect), and yet have not lost their vitality. It is sin- 

 gular that in this desiccated condition, they may be heated to a tem- 

 perature of 250, without the destruction of their vitality ; although, 

 when in full activity, they will not sustain a temperature of more than 

 from 112 to 115. Some of the minute Entomostracous Crustacea, 

 which are nearly allied to the Rotifera, appear to partake with them in 

 this curious faculty. Many instances are on record in which Snails 

 and other terrestrial Mollusca have revived, after what appeared to be 

 complete desiccation ; and the eggs of the Slug, when dried up by the 

 sun or by artificial heat, and reduced to minute points only visible with 

 the Microscope, are found not to have lost their fertility, when they are 

 moistened by a shower of rain, or by immersion in water, which restores 

 them to their former plumpness. Even after being treated eight times 

 in this manner, the eggs were hatched when placed in favourable cir- 

 cumstances ; and even eggs in which the embryo was distinctly formed, 

 survived such treatment without damage. That such capability should 

 exist in the animals and eggs just mentioned, shows a remarkable adap- 

 tation to the circumstances in which they are destined to exist ; since 

 were it not for their power of surviving desiccation, the races of Wheel- 

 Animalcules and Entomostraca must speedily become extinct, through 

 the periodical drying up of the small collections of water which they 

 inhabit ; and a season of prolonged drought must be equally fatal to 

 the terrestrial Mollusca. 



160. It would seem that many cold-blooded animals are reduced, by 

 a moderate deficiency of fluid, to a state of torpidity closely resembling 

 that induced by cold; and hence it is, that during the hottest and driest 

 part of the tropical year, there is almost as complete an inactivity as in 

 the winter of temperate regions. The common Snail, if put into a box 

 without food, constructs a thin operculum or partition across the orifice 

 of the shell, and attaches itself to the side of the box : in this state it 

 may remain dormant for years, without being affected by any ordinary 

 changes of temperature : but it will speedily revive if plunged in water. 

 Even in their natural haunts, the terrestrial Mollusca of our own cli- 

 mates are often found in this state during the summer, when there is a 

 continued drought ; but with the first shower they revive and move 



