ON THE EELAEION OV THE ACAETDjE TO THE AEACHNIDA. 279 



Some Observations on the Relation of the Acaridce to the Aracli- 

 nida. By Henet M. Beenaed, M.A. (Communicated by 

 A. D. Michael, F.L.S.) 



[Kead 21st April, 1892.] 



(Plate XX.) 



In trying lately to deduce the Copepoda from some primitive 

 form of Apus, which latter I had endeavoured to establish as 

 the probable racial form of the majoritj'' of the modern Crustacea, 

 I was driven to the conclusion that they could only be deduced 

 from the larval forms of such animals. I was compelled to take 

 this view because the organization of the Copepoda appeared to 

 be much lower than that of any of the forms which I consi- 

 dered primitive. It occurred to me at the time that the relation 

 of the Mites to the other Arachnids might perhaps be explained 

 on the same principle. The principle is not that the animals 

 degenerate, but that, in the struggle for existence, it has proved 

 adrantageous to many to remaiu at a larval stage, and thus 

 minute enough to be inconspicuous both for purposes of escape 

 and of attack. It may also be worth remarking that the more 

 minute an animal is, the more abundant, relatively speaking, is 

 the amount of organic matter at its disposal for purposes of 

 nutrition. 



It is true that, as the term is ordinarily used, this fixation of 

 a larval stage would be called " degeneration ; " but I think that 

 the word " degeneration " is hardly applicable in this connection, 

 and that it should be confined to those cases in which highly 

 developed organs and functions, once acquired, have been lost 

 owing to parasitism. Ifc seems to me that no one who has studied 

 the Mites, even in the most cursory manner, can believe that they 

 are degenerated Arachnids in this latter sense, although they 

 seem to have been generally called so. Many of them are active 

 predatory creatures, with no such easy-going habits of life as 

 would lead to degeneration ; while among the Acarids themselves 

 there are forms which have very clearly degenerated through 

 parasitism, not from the Arachnidan, but from the Acaridan 

 standard. I shall endeavour to show that this "degeneration" 

 from the Arachnidan standpoint is almost purely quantitative, 

 not qualitative. 



If, then, they are not degenerated Arachnids (i. e. assuming 



LTNN. JOUEN. ZOOLOUV, YOL. XXIV. 21 



