ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 311 



Dipneumonous spiders must have been derived from the pulmonary sac, 

 and not the reverse. 



The secondary tracheal tubes may arise anywhere on a tracheal 

 trunk, when required, and quite independently of the pulmonary saccules. 



The arguments in favour of the branchial origin of the lung-saccules 

 (from sunken-in gill-lamellae) appears overwhelming. The lung-books 

 of scorpions and spiders have arisen independently. 



Tracheae in Araneae have a diphyletic origin, from entapophyses and 

 from lung-saccules. There is no evidence of any sort to indicate that the 

 spinnerets of spiders were derived from sunken-in lung-books. 



Phylogeny of Tracheae in Araneae.* — W. F. Purcell suggests that 

 the saccules (hollow air-containing leaves) of the second pair of lung- 

 books have been converted into tracheal tubules in the common ancestor 

 of the Dysderidae, Oonopidae, and Caponiidae. The resulting tracheae 

 then increased in size, and, as the number of the leaves of the anterior 

 lung-books decreased in inverse ratio, the former became the principal 

 organs of respiration. 



The second pair of spiracles retained their position, or may even have 

 moved slightly forwards, and the conversion of the entapophyses into 

 tracheae could not take place here, and would, moreover, be quite un- 

 necessary. 



In the Caponiidae the anterior pair of lung-books were converted 

 into tracheae in a similar manner, but at a later period, and indepen- 

 dently of the conversion of the posterior pair ; but as the latter 

 already provided almost the entire body with tracheae, the anterior pair 

 did not further increase in size. In the second place, in the progenitor, 

 or progenitors, of the remaining tracheate spiders, the posterior lung- 

 books became reduced in size and effectiveness by the disappearance of 

 their saccules, accompanied by an increase in the number of the leaves 

 of the anterior lung-books. Further, the posterior spiracles became 

 approximated and united to a single spiracle and moved towards the 

 hinder end of the body, thereby causing the entapophyses of the tracheal 

 segment to elongate. In this condition the Filistatidae, Sicariidae and 

 Palpimanidae have remained, with slight modifications, such as the divi- 

 sion of the tracheal ante-chambers into branches in some forms. 



In the great majority of the families, however, the elongated entapo- 

 physes became transformed into a pair of medial tracheal trunks, thus 

 producing a tracheal system consisting of four simple unbranched trunks, 

 which is still found in some genera at least, in nearly all the families. 



A new factor having been introduced, viz. the presence of the respi- 

 ratory entapophyses lying in the large ventral sinus containing venous 

 blood requiring aeration, we accordingly find the second respiratory 

 segment again taking a prominent part in the respiration in many forms, 

 owing to the increase in size and the branching of the medial trunks, 

 accompanied ultimately by a corresponding reduction in the size of the 

 anterior lung-books, e.g. in the Attidae. This method of origin of the 

 tracheae is independent of that of the Dysderidae and their allies, and 

 the tracheal tubules, when present, would here not be derived from 

 saccules, but would be new formations. 



* Quart. Journ. Micr. Sci., liv. (1910) pp. 519-61 (1 pi. and 21 figs.). 



