Organs of Invertebrate A nimals . 39 



branchial office, the rule is for the upper one to do this 

 rather than the under one. Indeed, the case is one 

 which in some measure may serve to explain why the 

 leg of the prawn or other decapod should have at its 

 base an apparatus of gills and wisps and palps, or why 

 both parts of the leg should be transformed into lamelli- 

 form gills in the branchiopods. 



As between the leg and the gill of the decapod crus- 

 tacean, so also between the leg and the wing of the true 

 insect or hexapod, there is a very close relationship; 

 Oken divined their true nature when he spoke of these 

 wings as " aerial gills." The rudimental wings of the 

 pupae of certain water insects are true gills, acting in 

 every way like the very similar membranous and vascular 

 tegumentary expansions belonging to certain annelides : 

 and, as in the Pterophora, the true wing may be split up 

 longitudinally into rays so as to be no inapt representa- 

 tives of the tufted tergal gills of the nereis. Moreover, 

 the gills and the wings in these and other instances are 

 both of them developed from the same part of the 

 annular segment the tergal arc. There is, in fact, no 

 difficulty in admitting, not only that the wings are 

 " aerial gills," but also that the wing may be related to 

 the leg in the same way as that in which the gill is 

 related to the leg in the decapod crustacean. It is cus- 

 tomary to speak of the wings and legs of insects as 

 developed from different parts of the same segment, and 

 in the perfect insect, no doubt, a special connection is 

 traceable between the wings and the tergal arc of this 

 segment and between the legs and the ventral arc. In 

 fact, however, the wings and the legs are developed, in 

 close relationship to each other, on each side of the 

 segment, from the part which lies between the contiguous 

 ends of the tergal and ventral arcs, and the connection 



