OF THE ACAEID^ TO THE ABACHNIDA. 285 



general agreement in number and arrangement of appendages, 

 and upon certain unmistakable likenesses in inner organization. 



Of tbese last, perhaps the common possession of a tendinous 

 endosternite is the most striking. This structure, whose pro- 

 bable origin I have described in another place*, only occurs, 

 well developed t, in the Arachnida and in the archaic Crustacea, 

 Apus and Limulus. We have not hesitated, therefore, in follow- 

 ing the usual classification, and in claiming the Acarids as Arach- 

 nids throughout this paper. 



I may sum up the arguments which lead me to believe that 

 the Acaridse are Araneids fixed at a larval stage of development 

 because of the many advantages which animals of such small size 

 have over larger ones, as follows : — ■ 



(1) On comparing the segmentation of a simple form of Acarid 

 with that of an Araneid, seven abdominal segments in front of 

 the anal segment are not developed, while all the developed seg- 

 ments in the Acarids are easily homologized with the similar 

 number of anterior segments of the Araneid, segment with 

 segment. 



(2) We find, on comparing the alimentary canal of the Acarid 

 with that of an Araneid, that it is clearly the abdominal part of 

 the latter which is missing in the former. 



(3) The heart of the Acarid (as exemplified by Oamasus) is 

 clearly, both in shape and position, an Araneid heart arrested in 

 its development ; the abdominal extension of it is wanting. 



(4) The ventral ganglionic masses in both agree in almost 

 every point, the only difierence being due to the greater develop- 

 ment of the abdomen in the Araneid. 



(5) The size of the eggs of the Acaridse is often out of all 

 proportion to the size of the animals themselves ; this seems to 

 show that, while the animal has diminished in size, the egga 

 have retained more nearly the size of those of the original adult 

 forms. 



(6) A further argument, based on the position of the trachese, 

 will be adduced later on. 



If the principle by which we have attempted to explain the 

 origin of the Acaridse holds good, it is probable that it has 

 been of very general application in the development of the 



* ' The ApodidiB ' (Macmillan, 1892), pp. 56, 57. 



1" There can be little doubt that a rudiment of it persists in Astacus. 



