6 



ARKIV FÖR ZOOLOGI. BAND 7. N:0 28. 



dorsal shields into as few as possible, owing probably to tlie 

 ad ventages as regards the protection this offers. Still one 

 genus, Sejiis, exhibits a very primitive feature in this re- 

 spect, possessing 6 shields; in only few other genera this 

 feature is still visible during the ontogenetic development. 



In many genera the shields ha ve completely coalesced; 

 but also in these the ontogenetic development proves that 

 they are descendents from forms with two dorsal shields. 



Already Kramer [5. p. 388] suggested that it might be 

 possible for the shields of the Parasitidce to throw some light 

 on the question of the segmentation of the acari. He him- 

 self, however, pronounced the difficulties to be overcome too 

 great as yet. Since the time of the publication of Kramer's 

 paper, much has been written on this subject; and in spite 

 of opposite opinions by many scientists it must now be 

 looked upon as settled by Reuter's work that, when the 

 opistosoma is segmentated, this is a primitive feature, not a 

 secondary one. 



In Pediculopsis graminum (E. Reut.) Reuter counts 6 

 segments on the opisthosoma. 



In Sejus, where the anterior dorsal shield belongs to 

 the prosoma, the opisthomatic shields would be the rem- 



nants of 3 segments, the others being 

 wholy coalesced. 



In the few Uropodince, which ha ve 

 two dorsal shields, the posterior one 

 is generally very small in comparison 

 with the anterior one; and this seems 

 to signify that it is not homologuous 

 to the posterior one of other groups, 

 but only with the posterior one of 

 Sejus the anterior one being com- 

 posed of the anterior, prosomatic shield 

 which exists in Zercon for instance, 

 and the two pairs of small shields 

 which Sejus possesses. 



Fig. 10. Uropoda f orami- 



nifera Tgdh. $ (after 



Trägåkdh). 



