J. D. Dana on the Homologies of Insects and Crustaceans. 235 
Insect and Crustacean must be rightly given; consequently, if 
there is any doubt, it holds only with regard to Nos. 1, 2 and 8, 
he law of unity of structure under a type seems, however, to 
preclude even this chance for doubt. 
Comparing the higher Decapods among Crustaceans and the 
higher Insects, the mean size or mass is about as 50 tol, This 
ratio indicates approximately the amount of condensation in the 
Articulate structure connected with the elevation of grade from 
the typical Crustacean to the typical Insectean. | 
® Only in a degradational of Decapods, that of the Gastrurans, do the vis- 
cera reach re she phclonniesid oan nts, or those following the 14th. The abdomen 
is very much elongated in these species, the cephalothoracic portion of the body is 
comparatively small, and the whole structure is lax and low in grade. The species 
«thus stand apart from the Macrurans, as a separate tribe, equivalent with those 
Brach urans and Macrurans; hile the Schi i d y degradational acrurans. 
See this Journal, [2], xxv, 338. In the fact that the viscera of the Squilloids or 
Gastrarans are contained in the abdominal portion of the animal, this group ap- 
Pears to a the order of Insects. But this seeming approximation comes, as 
observed, Jegradation, and is analogous to that between a Limulus and an 
Insect, as explained on page 6 of this volume, 
