200 Hen- J. Wagner on the 



the dt'inial glands may become transformed into respiratory 

 organs *. 



It is shown by palaeontology that the Gigantostraca, which 

 stand very near to the ancestral forms of the Arachnids, 

 occur in the lowest strata of the Silurian system in typical 

 marine deposits, in the upper stratum already in company 

 with Phfjllocarida, Ostracoda, and Ganoid fishes, and, lastly, 

 in the productive strata of the Coal-measures among remains 

 of land-plants, together with those of Scorpions, Insects, 

 Fishes, and Ireshwater Amphibia. " We may therefore 

 assume," remarks Zittel (No. 70, p. 647), "that they lived 

 originally in the sea, and subsequently in brackish or perhaps 

 even in fresh water." Korschelt and Pleider (No. 31, p. 533) 

 therefoie regard it as possible that the Gigantostraca became 

 adapted not only to a freshwater life, but also to existence 

 upon land. With respect to these palseontological data, in 

 order to explain the development of two respiratory organs 

 of the Arachnids which differ in their origin, I find it a 

 very intelligible hypothesis to suppose that certain of the 

 ancestral forms of the Arachnida, all of which were closely 

 allied to the Gigantostraca, passed a certain period of the 

 year or a certain stage of their postembryonic development 

 out of the water; these forms (like the Gigantostraca) 

 possessed branchias completely covered externally upon 

 several of the anterior segments of the abdomen, and nume- 

 rous dermal glands, of which the excretory ducts, which were 

 of some length, gradually became filled with air after the 

 animals left the water, and so temporarily served as the seat 

 of the exchange of gases in the blood. 



The dermal glands of the Araclinids, after their function 

 became changed into that of respiration, may have undergone 

 further development in precisely the same manner as the 

 trachea} of the rest of the Tracheata, since the tracheai of 

 PerijjatuSj as well as the dermal glands, and consequently the 

 primitive trachege of the Arachnids, have been developed 

 from the same (mucus-secreting) dermal glands belonging to 

 the Annelid-like ancestors of all the Arthropods. From 

 these forms the Acarina, Solifugse, and probably also certain 

 other orders of Arachnids have arisen. Their fascicular f 



* For this reason it seems to me strange that Weissenborn states, in 

 ■combating the theory of the transformation of the gills of Limulus into 

 the lungs of the ticorpion, enunciated by Ray Lankester and MacLeod, 

 that, assuming such a transformation to have taken place, " the occur- 

 rence of the thoracic stigmata and ti-achese can only be regarded as a 

 formation of new structures which is difficult o explain." 



t I would here point out that the rosette-shaped arrangement of the 



