CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 13 



it ; the presence of Malpighian tubules ; the unpaired condition of 

 the reproductive organs of both sexes; and the fact that these organs 

 open into the intestine. The central nervous system consists of a 

 brain, a suboesophageal ganghon, and a ventral chain of four ganglia, 

 connected by widely separated connectives. 



The tardigrades are very abundant, and are very widely dis- 

 tributed. Some live in fresh water, a few are marine, but most of 

 them live in damp places, and especially on the roots of moss, growing 

 in gutters, on roofs or trees, or in ditches. 

 But although they are common, their 

 minute size and retiring habits result in 

 their being rarely seen except by those 

 who are seeking them. 



Many of them have the power of 

 withstanding desiccation for a long period. 

 This has been demonstrated artificially by 

 placing them on a microscopic slide and 

 allowing the mositure to evaporate 

 slowly. The body shrinks, its skin 

 becomes wrinkled, and finally it assumes 

 the appearance of a grain of sand in 

 which no parts can be distinguished. In 

 this state they can remain, it is said, for 

 years; after which, if water be added, 

 the body swells, assumes its normal form, 

 and after a time, the creatures resume 

 their activities. 



Regarding the systematic position of 

 this class of animals nothing definite can 



be stated beyond the fact that they are doubtless arthropods. Their 

 relationship to the other classes of arthropods has been masked by 

 degenerative modifications. They are placed here near the end of 

 the series of classes of arthropods, merely as a matter of convenience, 

 in what may be termed an appendix to the arthropod series, which 

 includes animals of doubtful relationships. 



Fig. 14. — A tardigrade (After 

 Doyere). 



