Phylogeny of the Arachnida. 289 



and a narrower and paler inner division, which Croneberg 

 " never succeeded in following to the end amid the tracheae 

 and hypodermic cells surrounding it." On reading this 

 description we involuntarily compare it with that of the 

 tracheal in Peripatus furnished by von Kennel : the latter 

 states that the walls of the trunk of the trachea are clothed 

 with a distinctly perceptible layer of chitin, but that those 

 of the actual tracheal tubules are very delicate, scarcely 

 distinguishable amid the surrounding tissues, and in all 

 probability do not consist of chitin, since they are easily 

 soluble in cold liquor potassge. We may further point out 

 that, according to Croneberg {loc. cit.) , the excretory duct of 

 the dermal glands of Eyla'is and other Hydrachnids, which 

 are situated in the cephalothorax " in front of and behind the 

 first segment of the fourth pair of legs, actually reminds us 

 slightly of the trunk of a trachea;" while according to 

 Furstenberg (No. 16, p. 192) the "oil-glands," which are 

 undoubtedly homologues of the dermal glands of the Water- 

 and other Mites, have the power of filling themselves with 

 air. The dermal glands of the Mites are, as is self-evident, 

 secondary structures, and it is not from these that the develop- 

 ment of the Arachnid tracheae which we are discussing has 

 proceeded ; yet it is perfectly natural to suppose, by analogy 

 with the Acarina, that the aquatic ancestors of the Arachnids, 

 after they were already provided with a firm chitinous 

 covering, also possessed numerous dermal glands of a similar 

 kind. Such an assumption, in my opinion already probable 

 in itself, is strengthened both by comparison with the dermal 

 glands of the Pantopoda, and also partly by data furnished 

 by paleontology. In the Pantopoda, as is well-known, 

 numerous dermal glands open upon the surface of the body, 

 which they entirely envelope in a viscid layer composed of 

 their secretion; as the chitinous covering is considerably 

 developed, the dermal glands are enclosed in special cavities 

 (caverns) within it. These cavities, which open by " pores " 

 upon the surface of the body, were regarded by Zenker 

 (No. 69, p. 383), and after him by Hoek (No. 21, p. 124), as 

 a special respiratory apparatus; yet the investigations of 

 Dohrn (No. 13) have proved that they are only receptacles of 

 the glands. A difference of opinion of this kind led Weissen- 

 born (No. 67, p. 110) to assert " that originally all these 

 cavities contained the dermal glands discovered by Dohrn, 

 but that a portion of them gradually became subservient to 

 respiration, since gas was secreted in the place of fluid." 

 This is regarded by the author in question as a proof that 



