442 



PHYSIOLOGY. 



and function perfectly agree with the ribs. But I have never been 

 able to find a second series of vertebral arches for the nervous cord. 



259. 



We must now proceed to the comparison and explanation* of the 



limbs. We have shown above that the wings are no limbs, but gills 



(243)." For the completion of this uniformity, which is already 



exhibited by their structure from two external membranous layers and 



the currents of blood distributed between them, we have only to show 



that these gills lie precisely where when present the gills are always 



found. If we, therefore, commence with the superior groups, we shall 



find that in them the branchial apertures lie at the anterior part of the 



neck, and, consequently, upon the ventral side. In some amphibia, 



namely, the Batrachians, they are found exactly at the same spot. 



The same in the fishes. In these they are protected additionally by 



means of a bony covering which opens posteriorly and beneath, and 



therefore towards the venter. In the Mollusca, the branchiae lie more 



or less upon the back, as it is called, but their back is nothing else than 



the true belly, from the very same reason, that in them also the main 



nervous mass takes its course along the so called ventral disc. 



We thus obtain this general law for the situation of the branchiae, 

 viz. that they are placed laterally, but inclining to the venter, arid 

 thence descend to the venter. The wings have the same position ; they 

 originate between the scapula and sternum, consequently upon the 

 external side of the body, and thence descend to the ventral side. In 

 those insects furnished with gills, they also lie at the same part ; they 

 always originate at the side where the main stems of the trachea? lie, 

 and thence rise upwards, either wholly so or partially inclining back- 

 wards, as in the larvae of the Ephemera and Phryganca>. As long as 

 the insect dwells in the water, its rudimental wings are true water gills, 

 but so soon as it has quitted the water, they transform themselves into 

 air gills, for in both cases fluids circulate in their vessels, which, 

 doubtlessly, receive oxygen from the air. The beetles exhibit the most 

 perfect conformity, for in the entire hardening of their body the wings 

 also have become horny, and now supplant the place of gill covers. 

 The Orthoptera approximate to this structure by their pergamentaceous 

 superior wings, but many Hemiplera approach still closer, for in them 

 half the wing is horny. Thence we must remove the wings from the 

 category of limbs, they being merely gills which are occasionally moved 



