848 MR JAMES MURRAY ON 



Other opinions had fewer adherents. Dujardin, 1841 (6), united them with the 

 Rotifers to form his class of Systolides, a classification hardly accepted by anyone 

 except Doyere, 1840 (4), who afterwards abandoned it. Dujardin himself, in 1851 

 (7), admitted that they could not be classed with Rotifers, but contends that neither 

 are they near the Acari. 



Graff (15) makes a special order, Stelechopoda, of the Myzostomida, Linguatulida, 

 and Tardigrada. 



Plate, 1888 (30), regards them as the lowest of the Tracheata, near the 

 Onychophora. 



Lance, 1896 (20), places them between the Worms and the Tracheata, in the Pro- 

 tract) eata, near Peripatus. 



Greeff, 1865 (17), admits that the general opinion places them with the Acari, 

 and gives reasons against doing so, without committing himself to any more definite 

 opinion. 



Basse, 1905 (1), denies any close affinity with Peripatus, and places them again in 

 the Tracheata. 



The fact that so many good zoologists have supported such different views of the 

 affinities of the Tardigrada appears to indicate that their essential structure does not 

 incline very markedly to any one group of the Arthropoda more than another. They 

 form themselves a very distinct group. 



This being so, such subordinate characters as the possession of four pairs of limbs, 

 the absence of distinct abdomen, and the simplicity of the circulatory and respiratory 

 arrangements, gain weight in indicating an affinity with the only other Arthropoda 

 similarly characterised, viz. some of the lower Mites. 



That the affinity is not really very close is, I think, indicated by the fact that, 

 although the adults possess four pairs of limbs, they do not, like the Acari, at any stage 

 in their development possess only three pairs. 



We find among Tardigrada and certain Acarina a retrogression occurring at a certain 

 stage in development, or under certain conditions, which results in an encystment 

 having a marked analogy with that of such common occurrence among Protozoa (with 

 which Megnin compares it), and which, outside of these two groups, has no known 

 parallel among animals higher than the Protozoa. 



The remarkable coincidence of even the secondary details of the process in 

 Macrobiotus dispar and Glycophagus domesticus, even to the final leaving the cysts 

 by a trap-door, can hardly be regarded as other than fortuitous. 



The essential part of the process, however, — the formation of cysts, within which 

 the animals return in a greater or less degree to an amorphous condition, — seems to me 

 to strengthen the belief that there is a real affinity between the two groups. 



