60 J. MURRAY ON WATER-BEARS, OR TARDIGRADA. 



There is a mucus-gland in each foot, and an apical pore 

 connected with it, opening at the end of the foot (see Fig. 9). 



The sexes are distinct ; but the male and female sexual organs 

 are very similar in appearance. The males are much less 

 numerous than the females. 



The skin is frequently shed, and in the majority of species the 

 eggs are laid during the moult, and are left enclosed in the 

 old skin as a protective case. When this is done, the eggs 

 are smooth. In many species of the genus Macrobiotus the eggs 

 are laid free of the skin, and such eggs are always covered with 

 spines or tubercles. 



The Water-bears possess, in common with the Bdelloid Rotifers, 

 the power of reviving after prolonged desiccation. 



They have the further power of withstanding asphyxiation. 

 In the normal healthy, active animal the nerves and muscles can 

 hardly be detected, but in the asphyxiated animal they stand out 

 distinctly. For the study of the internal organisation it is, 

 therefore, desirable to have them in this condition, and it may 

 be easily produced by placing the animals in boiled water and 

 excluding the air by a layer of oil. 



Geographical Distribution. 



The Tardigrada are what Professor Jennings, in speaking of 

 the Rotifers, calls " potentially cosmopolitan" — that is to say, there 

 is no barrier to their distribution over the greatest distances, and 

 the same species is likely to occur anywhere over the world under 

 similar conditions. 



A number of species are of world-wide distribution, being 

 found in the two polar regions and in all the continents. 



They do not, however, appear to be so adaptable as Rotifers, 

 and whatever the bar may be to their distribution, many species 

 have a very limited range. This restriction lends interest to the 

 pursuit, as every region has its own peculiar Tardigrade fauna. 



Systematic Position. 



A few years after Goeze discovered his " Wasserb'ar," the great 

 Danish naturalist, O. F. Miiller, first bestowed a scientific name 

 upon a species, w T hich he called Acarus ursellus (14). As the 



