J. MURRAY ON WATER-BEARS, OR TARDIGRADA. 187 



. With three other species the case is different. M. dispar and 

 M. amhiguus may have one pair of bhint knobs, which may be 

 regarded as homologous with e. M. aculeatus (25), an Australian 

 species, has three pairs of soft spines which may well be supposed 

 to occupy the positions of c, d and e, as was first pointed out to 

 me bv Prof. Richters in a letter. 



The indications of relationship which have been reviewed in 

 the preceding paragraphs are certainly somewhat complex, and 

 not a little conflicting, but nevertheless something may be drawn 

 from them of the possible genesis of the genera and species. The 

 genera cannot be arranged in series ; each is linked up with 

 several others. 



The time is premature for attempting to trace the genesis 

 new forms are constantly being discovered which alter our ideas 

 of the affinities still, some suggestions possible in the present 

 state of our knowledge may be permissible. 



It need not be supposed that any genus is directly in the line 

 of descent from the hypothetical primitive tardigrade to the 

 other diverse genera. Reviewing the ten known genera, it 

 appears that most of them have some degree of resemblance to 

 the well-known and extensive genus Echiniscus. Milnesium and 

 Maci'ohiotus, with its dependent Diphascon, are furthest removed. 



All the marine genera, and Oreella, have the head-processes 

 traceable to modifications of the type of Echiniscus, and they 

 further conform to it in the structure of the teeth and pharynx 

 and in having all the claws similar and separate (the barbs of 

 Echiniscus itself being only a trifling departure from the 

 uniformity, and the claws of Batillipes differ only in size). 



As Echiniscus is such a greatly preponderant genus among 

 the Echiniscoid genera (all the others possessing at present only 

 one species each), and as they are all highly specialised animals, 

 it may be legitimately supposed that from a dominant type, not 

 unlike Echiniscus, these have been derived by modification in 

 adaptation to the peculiar situations in which they now live. 

 Oreella is in a rather different position to the others, occurring 

 as a moss-dweller under conditions comparable to those in which 

 the great majority of both Echinisci and Macrohioti dwell. It 

 may be that Oreellci can tell us more of the line of descent than 

 any of those more specialised animals. In fact, it stands nearer 



