42 ME. M. LAIJKIE ON THE 



In the Dipneumones the posterior pair o£ lung-books are replaced 

 by tracheae which, according to this view, have developed by 

 an extension of the air-chamber of the lung-sac, as has been 

 suggested by Macleod*. 



Two pairs of abdominal appendages seem to be converted into 

 spinning mammillae in the Araneina. In LipMstius they occupy 

 a normal position on the ventral surface of the abdomen, but in 

 the higher forms, in which the segmentation of the abdomen has 

 been entirely lost, they are shifted to a posterior position. This 

 accounts for five appendages of the abdomen, which is all that 

 seem to appear in the embryo. It may be advanced as an argu- 

 ment against my view, that if we consider the second lung-book 

 as belonging to the fourth abdominal segment instead of the 

 third, then we have, with the spinning mammillae, all six abdo- 

 minal appendages accounted for ; but it seems to me more likely, 

 without considering other reasons, that the sixth appendage has 

 vanished than that the second has disappeared without leaving 

 any trace. 



In the other Arachnids the lung-books are replaced by 

 tracheae. Of Galeodes, the pons asinorum of all who have tried 

 to deal with Arachnid morphology, I do not intend to speak here. 

 The presence of stigmata leading into tracheae between the 

 fourth and fifth thoracic appendages is perplexing, not to say 

 bewildering. I fully agree with Bernard in considering this 

 form of great importance, though I do not feel convinced of its 

 being primitive in most respects. We must wait, however, till we 

 have a more careful and detailed account of its anatomy than has 

 yet been published before we can speculate as to its morphology 

 with any hope of success. 



It has often been maintained that the lung-books of Arachnida 

 are derived from tracheae and not from branchiae ; but this view 

 cannot, I think, be accepted. The fact that lung-books are cha- 

 racteristic of the two most primitive orders — the Scorpions and 

 the Pedipalpi — while in the Spiders, in which both are present, it 

 is the higher forms — the Dipneumones — which have tracheae, 

 affords a strong argument against it. It is said that the inde- 

 pendent development of tracheae so closely resembling each other 

 in the Insects and Arachnids cannot be thought of as possible; but 



* Arch, de Biol. vol. v. 



