288 Ilerr J. Wagner on the 



the position of the stigmata is to be explained by the character 

 or the degree of the development of the corresponding seg- 

 ments of the body. 



If the ce])halothoracic trachea? have arisen within the limits 

 of the Arachnida, which are Arthropods of an already fairly 

 well-developed type, we are confronted with a second question 

 as to the organs whence, and the manner in which, they may 

 have been evolved. Von Kennel thiidcs (No. 25 ; and No. 24, 

 p. 25) that the trachea of Peripatus, and consequently of the 

 lii'-'hcr Tracheata also, may be derived from unicellular 

 (mucus-secreting) dermal glands of Annelid-shaped ancestors 

 of Feripai7is ; this view was previously expressed by 

 Moseley (No. 49), and harmonizes with the circumstance 

 that the stigmata in Peripatus Edicardsii, Blanch., are distri- 

 buted over the entire body, as also with the structure of the 

 trachese of the Onychophora, in consequence of whicli it has 

 received of late general recognition [cf. Lang's ' Lehrbuch,' 

 No. 38, p. 451). In precisely the same manner we may 

 exidain the origin of the tracheee in the cephalothorax of the 

 Arachnids : they develop likewise from unicellular dermal 

 glands, which were inherited from the same Annelid-like 

 ancestors of the whole of the Tracheata, which explains the 

 apparent similarity in structure between the trachea3 of the 

 Arachnida and of the rest of the Tracheata. We also still 

 tind in the aquatic Arachnids a large number of hypodermic 

 glands, w hich usually open in pores at the base of the setse ; 

 these glands are developed in the Water-mites, and were 

 regarded by the earlier observers (Dujardin) as sacs, serving 

 as respiratoiy organs. Approximately the same view has, as 

 is stated by Croneberg (No. 11, p. 8), been expressed by 

 Claus in his text-book, and perhaps it was this latter circum- 

 stance which occasioned the statement on the part of Lang 

 (No. 38, p. 550) that " in certain Acarina short tubes or 

 pouches in connexion with orifices in the external chitinous 

 covering have been interpreted as rudiments of trachete." 

 According to the papers and evidence of Clapar^de (No. 10, 

 p. 463), Croneberg (No. 11, p. 8), Koenike (No. 30, p. 357), 

 von Schaub (No. 56, pp. 110, 111), and others, the glandular 

 character of these structures admits of no doubt. It appears 

 from the majority of the statements that these glands are 

 destined by means of their secretion to preserve the integu- 

 ment of the Water-mites from drying up on leaving the water 

 (f/. the function of the "oil-glands" of the Cheese-mites 

 according to Nalepa, No. 50, p. 205). In Eyla'is the excre- 

 torv ducts of the glands consist, according to Croneberg, of 

 two sections — a peripheral, more strongly chitinized portion, 



