2^0 KINGS LEY. [Vol. VIII. 



Cases like this clearly show us that tracheae may arise in 

 different ways from different sources. 



The presence of the so-called spiral thread in the tracheae of 

 both Arachnids and Hexapods has been adduced as an argument 

 in favor of the homology of the organs in both groups. As in 

 both cases the tracheae are formed as invaginations of the 

 external integument it is natural that they should consist of 

 tubules of ectoderm lined with a chitinous intima, and the 

 thinner this intima the easier the transfer of gases through it. 

 But if it become too thin the tube is liable to total collapse by 

 the pressure of the various viscera upon it and so the chitinous 

 layer is developed into folds or corrugations, which when regu- 

 larly arranged form the spiral "threads." In many spiders they 

 are not regular and show clearly their origin in response to the 

 mechanical conditions presented. 



The greatest difficulty in connection with this view of the 

 origin of tracheae from gills through the lungs is that presented 

 by the Solpugids and certain Acarina where tracheal stigmata 

 occur in the cephalothoracic region, where they should not 

 occur according to our thesis. Yet until we know more of 

 the structure and ontogeny of these organs the full weight 

 of this objection cannot be properly estimated. A full his- 

 tory of Solpuga would settle many questions of arthropod 

 morphology. 



To summarize : The lungs of the scorpion arise in the same 

 way and on the same somites as the gills of Limulus. In the 

 one they sink into a pit, in the other they remain free. The 

 homologies between the lungs and tracheae of Arachnida were 

 demonstrated by Leuckart. Hence the lungs, the fan tracheae 

 of authors, are to be regarded as the primitive, the bush-like 

 the derived form, and these tracheae have no relation to those 

 of Hexapods. 



The comparisons between the nephridia of Limulus and 

 those of the Arachnida have been made upon a previous page. 

 The observations made by Laurie, Kishinouye, and myself have 

 clearly shown that we have to deal here with structures homol- 

 ogous with the nephridia of the worms, although but a single 

 pair may persist in its unmodified condition. We find, how- 



