or THE ACAETD^ TO THE AEACHISTIDA. 283 



The nervous system also offers slight confirmatory evidence. 

 rig. 5 (A, B) compares the nervous systems of an Arachnid {My- 

 gale, A) and of a Mite {Gamasus, B). Only the great ventral 

 ganglionic mass is developed in the latter. All its great diverging 

 nerves correspond vrith those from the ventral ganglion of My- 

 gale. When we reach the abdominal nerves, however, we find that 

 in Gamasus they branch out direct from the sternal mass, while 

 in the Araneid, in accordance with the form of the animal, they 

 run together in a long strand (to swell in some cases, as in 

 Mygale, into an abdominal ganglion) into the abdomen, and there 

 branch out among the viscera. 



The arguments here brought forward to show that the Acarids 

 are larval Araneids receive some slight support from the fact 

 that as a general rule in the Acarids the eggs are of an enor- 

 mous size as compared with the parent animal. Although the 

 Acarid stopped growing long before reaching the full number of 

 segments of its adult original form, there was no reason why the 

 eggs developing from the germinal epithelium should diminish 

 in size in correspondence with the altered size of the parent. 

 The size of the egg would of course gradually be fixed for each 

 species by natural selection. It is, however, significant to find 

 that, as a rule, the eggs of the Acaridge are enormous, rela- 

 tively, that is to say, to the size of the animals. This seems 

 to indicate that the level of organization in the Mites has 

 been well sustained, and that the alteration is almost entirely 

 one of size. 



These few comparisons between the internal structure of the 

 Mites and that of other Arachnids (chiefly Araneids) lend con- 

 siderable support to the evidence afibrded by the segmentation, 

 that the former are Araneids arrested in their development, i. e. 

 that they are fixed larval forms. 



The foregoing argument, it is clear, gains much in strength if 

 it can be shown that Tetranychus can really claim to be a primitive 

 form. I think that a study of the mouth-parts leaves little 

 doubt on this point. 



The mandibles have been somewhat telescoped into the body 

 and are fused together, hut their outlines are still clear (fig. 1). 

 Although in Tetranychus tiliarum their distal joints have been 

 modified into long retractile piercing-tubes, in another appa- 

 rently allied species described by Elogel as parasitic on Spiders (in 

 whose body its bite gives rise to a curious dendriform tissue) the 



