310 IIl'ii- J. Wagner on the 



tracheae is contradicted by the circumstance that it is precisely 

 ill the Acarina, which are distinguished from tiie rest of the 

 Arachnida by the position of the stigmata, tliat tlie tracheae 

 do not appear before the second postembryonic period ; in the 

 embryos and the hexapod hirvse they are totally wanting. In 

 the given case, too, as in the speculation on the origin of the 

 tracheae, Bernard's reasoning is weak, and his criticism of 

 already firmly established facts is insuflicient or fails entirely. 

 As I have already stated, like the majority of naturalists, I 

 regard the Acarina as highly degenerate forms, whose 

 degeneration is expressed mainly in the following peculiarities 

 which they exhibit: — (1) The body is unsegmented, and all 

 its sections are fused into one ; (2) in connexion with this 

 character the muscles of the body-walls have undergone 

 degeneration ; (3) the abdominal section is relatively insig- 

 niticant ; (4) in certain Acarina the claws on the legs are 

 wanting; (5) the heart, when present, is of a peculiar form, 

 in other cases it is absent ; (G) the eyes are feebly developed 

 or entirely wanting ; (7) the coxal glands are absent ; (8) the 

 endosternite is wanting in many cases ; (9) in certain cases 

 the trachea are absent. Other characteristic features of the 

 Acarina, such as the powerful development of the dorso- 

 ventral bundles of muscles, the complete centralization of the 

 nervous system, the exclusively thoracic position of the 

 stigmata, the great development of the so-called Malpighian 

 tubes, the form of the alimentary canal, the peculiar hexapod 

 larval stage, and, finally, the great diversity of the forms, 

 point to the fact that the Acarina are an extremely specialized 

 type, and likewise deserve attention in analyzing their rela- 

 tions to the other orders of Arachnids. 



Witli reference to this latter question, as also to tlie mutual 

 relations of the Arachnidan groups in general, it must be 

 admitted that our knowledge of the development, and even of 

 the structure of the individual groups, is too slight to enable 

 us to institute such comparisons upon a sound basis. It is 

 only as a provisional theory that we can advance the one or 

 the other view as to the relations of the various groups of 

 Arachnids to one another. No greater assistance for the 

 decision of the question is afforded by the discoveries of 

 palaeontology, since in the case of the majority of Arachnids 

 exceptionally favourable conditions are needed for the preser- 

 vation of the soft parts; this, too, is probably the explana- 

 tion of the fact that, e. g. in the Mesozoic strata, fossil 

 Arachnids (with the exception of a single doubtful specimen) 

 are entirely wanting ; feolifugse are entirely unknown as 

 petrifactions, on the other hand Acarina, Pseudoscorpions, 



