Morphology of the Arachnida. 7 



the mouth. Tliis procession of tlie abdominal sternites is 

 accompanied by the obliteration of tiic anterior pair of tracheal 

 stigmata with the sternite that supported them. The remain- 

 ing stigmata are situated on the tirst free sternite, and their 

 presence fixes this phite as belonging morphologically to the 

 fourth somite *. Since, however, there is an immense range 

 in structure within the Opiliones, especially touching the 

 number and distinctness of the segments of the abdomen, it 

 is not always easy to compare the somites with those of the 

 Pedipalpi or Chelonethi. 



In Stj/Iocelhis, however, a genus in which the segmentation 

 of the abdomen is well developed, there are nine distinct 

 tergites and eight sternites, and since the presence of the 

 stigmata on the first sternite points to it morphologically as 

 the fourth, we have almost the full complement of somites 

 from the fourth backwards. Moreover, between the last 

 tergite and sternite there is a single anal sclerite which may 

 represent the twelfth tergite. The first free tergite has no 

 free sternite to correspond to it ; it appears to belong morpho- 

 logically to the third somite, since it immediately precedes 

 the tergite that covers the sternite upon which the tracheal 

 organs open. In this case the dorsal elements of the first 

 two somites have disappeared, and on the ventral side the 

 first three sternites appear to be represented in a general way 

 by that part of the intercoxal area of the cephalothorax 

 which lies behind the generative aperture. 



In most forms of Opiliones, however, the segmentation of 

 the body is less well expressed than in Styhcellus. This 

 genus appears in this respect, as well as in the small number 

 of supernumerary maxillary sclerites, to be one of the most 

 primitive of the order. In the higher forms the abdomen is 

 much more reduced in length, and even when distinct tergites 

 and sternites are strongly developed, as in e. g. the Laniatores^ 

 their number may fall as low as five ; while in others, such as 

 Phalangium, the segmentation is represented merely by folds 

 of the integument, the tergal and sternal plates being un- 

 developed. This reduction in the number of the dorsal and 

 ventral plates is brought about by their fusion with each 

 other or with the cephalothorax. 



That the Acari are nearly related to the Opiliones seems 

 evident from a comparison between the two orders. For, as 

 Dr. Thorell says, " from the Acari the Opiliones are scarcely 

 in all cases distinguishable by any other external character 

 than the structure of the abdomen, which, in the Opiliones, is 



* If the third, it is uot easy to explain the disappearance of the second 

 pair of btiguiata. 



