104 F. M. BALFOUR. 



General Conclusions. 



Without attempting to compare at length the develop- 

 ment of the spiders with that of other Arthropoda, I propose 

 to point out a few features in the development of spiders, 

 which appear to show that the Arachnida are undouhtedly 

 more closely related to the other Tracheata than to the 

 Crustacea. 



The whole history of the formation of the mesoblast is 

 very similar to that in insects. The mesoblast in both groups 

 is formed by a thickening of the median line of the ventral 

 plate (germinal streak). 



In insects there is usually formed a median groove, the 

 walls of which become converted into a plate of mesoblast. 

 In spiders there is no such groove, but a median keel-like 

 thickening of the ventral plate (PI. IX, fig. 11), is very pro- 

 bably an homologous structure. The unpaired plate of 

 mesoblast formed in both insects and Arachnida is exactly 

 similar, and beomes divided, in both groups, into two bands, 

 one on each side of the middle line. Such differences as 

 there are between Insects and Arachnida sink into insigni- 

 ficance compared with the immense differences in the origin 

 of the mesoblast between either group, and that in the 

 Isopoda, or, still more, the Malacostraca and most Crustacea. 

 In most Crustacea we find that the mesoblast is budded off 

 from the walls of an invagination, which gives rise to the 

 mesenteron. 



In both spiders and Myriopoda, and probably insects, the 

 mesoblast is subsequently divided into somites, the lumen of 

 which is continued into the limbs. In Crustacea mesoblastic 

 somites have not usually been found, though they appear 

 occasionally to occur, e.g. Mysis, but they are in no case 

 similar to those in the Tracheata. 



In the formation of the alimentary tract, again, the 

 differences between the Crustacea and Tracheata are equally 

 marked, and the Arachnida agree with the Tracheata. 

 There is generally in Crustacea an invagination, which gives 

 rise to the mesenteron. In Tracheata this never occurs. 

 The proctodsDum is usually formed in Crustacea before or, 

 at any rate, not later than the stomodgeum.^ The reverse is 

 true for the Tracheata. In Crustacea the proctodaeum and 

 stcmodseum, especially the former, are very long, and usually 

 give rise to the greater part of the alimentary tract, while 

 the mesenteron is usually short. 



* If Grobben's account of the development of Moina is correct this 

 statement must be consiuered not to be universally true. 



