254 



ZOOLOGY 



phylum. They may be quite soft-bodied and legless, 

 so that on inspection one would never suspect their 

 relationships. In such cases the zoologist is guided by 

 the general structure, which serves to indicate relation- 

 ships with more typical or ordinary forms. In many 

 groups degenerate members are found living a seden- 

 tary or parasitic life, and losing the striking peculiarities 

 which were developed by their ancestors. 



TheTardi- A very singular and perhaps really primitive type of 

 water bears arthropod is the so-called water bear. Water bears or 

 Tardigrada are microscopic animals found in ponds and 

 ditches, along with Protozoa, rotifers, and small worms. 

 They are transparent, more or less cylindrical, but fairly 

 stout, with four pairs of short, stout legs which are not 

 very different from the parapddia of worms. There is 

 a simple alimentary canal, but there are no well- 

 developed mouth parts or definite breathing organs 

 or blood system. The muscular fibers of the body and 

 appendages are unstriated, as in the case with muscles 

 not under voluntary control. Each individual carries 

 the organs of both sexes, and is 

 therefore said to be hermaphro- 

 ditic. 



What are we to think of such 

 a type ? Is it really a primitive 

 form allied to the worms, a-nd 

 surviving from a bygone age ? Or 

 is it a degenerate descendant of a 

 more highly developed ancestor ? 

 In any event, it is 'today an iso- 

 lated group, regarded as more or 

 less related to the mites, but 

 really without cousins in this 

 world. The species seem to be 



Drawing by W. P. Hay 

 FIG. 69. Echiniscus, one of the 

 Tardigrada; greatly enlarged. 



