Phylogeny of the Arachnidd. 289 



and a narrower and pak-r inner division, which Croneber/:^ 

 " iK'Vcr succeeded in followin;:^ to the end amid the tracheae 

 and hyjjodermic cells surrounding it." On reading this 

 description we involuntarily compare it with tliat of the 

 tracheaj in Peripntus furnished by von Kennel : the latter 

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

 with a distinctl}- jierceptible layer of chitin, but that those 

 of the actual tracheal tubules are very delicate, scarcely 

 distinguishable amid the surrounding tissues, and in all 

 jirobability do not consist of chitin, since they arc easily 

 soluble in cold liquor potassae. 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 tlie trunk of a trachea;" while according to 

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

 undoubtedly homologues of the dermal glands of the Water- 

 and other Aiites, have the power of filling themselves with 

 air. Tiie dermal glands of the Elites are, as is self-evident, 

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

 ment of the Arachnii> traclicai 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 botii by comparison with the dermal 

 glands of the Pantopoda, and also partly by data furnished 

 by palaeontology. 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 sjjecial respiratory apparatus ; yet the investigations of 

 Dohrn (No. 13) have proved that they are oidy recej)tacles of 

 the glands. A ditfcrence 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 



