MOEPHOLOGT OF THE PEDIPALPI. 43 



if we attempt to begin from tracheae we find that lung-books, more 

 closely resembling each other, have to be independently developed 

 twice, or more probably three times, so we are not much advanced. 

 Further, the similarity between the tracheae of Arachnids and 

 Insects has been much overrated. It seems to depend mostly on 

 the spiral thickening, which is present in both cases : but a 

 thickening of some sort is evidently a mechanical necessity in 

 these structures, and also the " spiral " is very poorly developed 

 in many Arachnids. The difference of position, too, must have 

 some morphological significance, — the tracheae of Insects &q. 

 arising outside the attachment of the appendages, while those 

 of Arachnids are inside. Bernard * would derive the tracheae of 

 both forms from setiparous sacs, and makes a great point of the 

 thoracic stigmata of Galeodes. Galeodes is a difficult problem, 

 whichever view we take, but far too little is known of its anatomy 

 (and still less of its development) to make it a safe ba^iis for 

 generalizing from. It is to be hoped that Dr. Bernard's forth- 

 coming paper on this form will give us some surer ground on 

 which to base our speculations. He talks of the " fascinating but 

 seductive " hypothesis that the lung-books are derived from 

 branchiae ; but it seems to me that a plentiful supply of seti- 

 parous sacs, capable of developing at will into lung-books, 

 tracheae, or coxal glands, affords a still more " fascinating " 

 hypothesis, and is, I am afraid, equally seductive. I do not 

 think that, in face of the development of the lung-books in 

 Fhrijnus, where they evidently arise as foldings of the posterior 

 wall of an appendage, it is possible to entertain the idea that they 

 are derived from setiparous sacs, and they do not seem to give 

 much indication of being derived from tracheae. It is unfortunate 

 that the development of the tracheae in Arachnids has never been 

 fully described, for I cannot but think that it would give some 

 indication as to whether they are primitive or derived from lung- 

 books f. 



The Coxal Gland. 

 There can be little doubt now but that this structure is 

 morphologically a nephridium. It has been shown to develop in 

 part from the mesoderm, and in the earlier stages to open into 



* Zool. Jahrb. vol. v., and Ann. & Mag. N. H. vol. xi. 1S93. 



t Vide Simmons, Am. J. Sci. Nat., Aug. 1894, and Ann. & Mag. N. H., Sept. 1894. 



