692 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 



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. 31, fig. n), is very probably an homologous 

 structure. The unpaired plate of mesoblast formed in both 

 insects and Arachnida is exactly similar, and becomes divided, 

 in both groups, into two bands, one on each side of the middle 

 line. Such differences as there are between Insects and Arach- 

 nida sink into insignificance compared with the immense differ- 

 ences 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 occa- 

 sionally 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 differ- 

 ences between the Crustacea and Tracheata are equally marked, 

 and the Arachnida agree with the Tracheata. There is gene- 

 rally in Crustacea an invagination, which gives rise to the 

 mesenteron. In Tracheata this never occurs. The proctodasum 

 is usually formed in Crustacea before or, at any rate, not later 

 than the stomodaeum 1 . The reverse is true for the Tracheata. 

 In Crustacea the proctodaeum and stomodaeum, especially the 

 former, are very long, and usually give rise to the greater part 

 of the alimentary tract, while the mesenteron is usually short. 



In the Tracheata the mesenteron is always considerable, and 

 the proctodaeum is always short. The derivation of the Mal- 

 pighian bodies from the proctodseum is common to most Tra- 

 cheata. Such organs are not found in the Crustacea. 



With reference to other points in my investigations, the 

 evidence which I have got that the chelicerae are true postoral 

 appendages supplied in the embryo from a distinct postoral 



1 If Grohhen's account of the development of Moina is correct this statement must 

 be considered not to be universally true. 



