280 ME. H. M. BERNARD ON THE RELATION 



tliem for the moment to be Araclmids) , we must explain their 

 minute size on some other principle, for, whether we adopt the 

 Eurypteriae or the more directly Annelidan origin of the Arach- 

 nida, we can hardly doubt that the primitive Arachnid was of much 

 more imposing proportions than these microscopic creatures. 



I propose in this paper to see whether it is possible to ex- 

 plain the origin of the Mites as I have endeavoured to explain 

 that of the Copepoda*. I shall try to show that the Mites are 

 not degenerated Arachnids, but Arachnids permanently fixed at 

 a larval stage of development. 



The establishment of this supposition appears, at the outset, 

 to be by no means easy. Many of the Mites are very highly spe- 

 cialized, and reveal little as to their earlier morphology. Acci- 

 dent, however, threw in my way a form which seems to be rather 

 primitive and to throw some light on the subject. I refer to the 

 common Tetranyclius tiliarum (Herm.), which I found swarming 

 in countless thousands down a lime-tree last autumn, carefully 

 keeping on the sheltered side of the tree. 



This Mite is morphologically of importance, because its seg- 

 mentation can be clearly made out, and it is on this naturally 

 that one chiefly relies for the explanation of its morphology. 



Eig. 1 (PL XX.) is a dorsal view of the animal, showing the 

 arrangement of the hairs, the dorsal muscles, and the three seg- 

 mental furrows and the relation of these furrows to the legs. 



The muscles would, I think, clearly show that these two 

 middle regions are true segments, even if the limbs did not make 

 this evident. I feel justified, therefore, in diagrammatizing the 

 segmentation of Tetranyclius in the way show in fig. 2 B f. 



We have the typical six segments of the Arachnidan cephalo- 

 thorax (leaving out of consideration the possible vanished antennal 

 segment), followed by three abdominal segments. The last or 

 third segment, as we should expect, contains the anal aperture ; 

 just in front of the anal aperture is the genital aperture, whether 

 on the first or second segment I cannot say ; I have marked it 

 between them. 



"When we compare this with the diagrammatic segmentation of 

 an Arachnid (fig. 2 A), we find that the two agree very closely, 



* ' The ApodidfE.' Macmillan, 1892. 



t We here assume that the mouth-parts of the Mites in general are purely 

 Arachnidan, as they most clearly are in Tetranychus. We shall, however, 

 return to this point. 



