WINGS OF INSECTS. 55 



that of the two columns of the spinal cord of 

 vertebrata. 



According to my demonstration of the true 

 ventral aspect also, of arthropods, it is evident that 

 the so-called 'dorsal' vessel is not dorsal, but really 

 thoraco-ventral, and corresponds with the heart of 

 vertebrate animals not only in function, but also in 

 relative position is, in fact, homologous with it. 



In both the vertebrate and arthropodous animal, 

 the trunk of the body is a chain of segments in the 

 former represented by the bodies of the vertebrae, 

 in the latter by the rings of the external skeleton. 

 But, even in arthropods, we discover evidences of 

 an internal segmentation in the chain of ganglia 

 forming the real posterior or sensitive column of their 

 spinal cord. 



Though the articulated extremities of arthropods 

 are productions of their external skeleton, and as 

 such may be compared to productions of the external 

 or dermo-skeleton of vertebrate animals, like the 

 dorsal, caudal, and anal fins of fishes, the same view 

 cannot, perhaps, be taken of the wings of insects. 

 It is worthy of observation that the wings of insects 

 correspond to the limbs of vertebrate animals in 

 relative position and in number, being turned to- 

 wards the real ventral aspect of the body and being 

 never more in number than two pairs, sometimes 

 there is only one pair ; and as there are vertebrate 



