12 



ARKIV FÖR ZOOLOGI. BAND 7. N:0 28. 



the question, which of these types is the mest primitive, or 

 whether the presence of peritremata is more primitive than 

 the absence of it; but I am inclined to beheve that the ab- 

 sence of peritrema is looked upon as being due to a retro- 

 gressive development as, indead, I myself did not long ago. 

 If we consider at first the undulating type of the JJro- 

 podince, it becomes at once evident that it is a recently ac- 

 quired feature, which is diie, if not entireJy, then at least 

 for the greater part, to the development of the pleural ridges 



Fig. 20. 



Fig. 19. Zercon curiosus Tgdh. $, ventral view X 148 (after Teägårdh), 

 Fig. 20. Dinychus tetraphyllus Berl. $, side view X 148 (after TrägIrdh). 



which accompanied the formation of the so-called fovealse 

 pedales, which serve the purpose of providing the legs with 

 a safe shelter. These high ridges were obstacles in the way 

 of the peritremata, which were forced to double round them, 

 and thus become more or less zic-zac shaped. 



The short type and entire absence of the peritrema are 

 also largely secondary, recently acquired features. It is, 

 however, in this respect necessary to draw a limit between 

 the parasitic and the free-living genera. 



