66 COMPARATIVE PHYSIOLOGY 



making use of the fact that the flagellate Bodo migrates to a 

 region having a certain optimum oxygen tension. Fox found 

 that when pupae of Simulium, which respire by means of 

 filamentous appendages at the junction of head and thorax, 

 are placed in a suspension of Bodo, the micro-organisms collect 

 at first round the filaments in a dense congregation, and then 

 migrate outwards in a crescentic configuration, as the oxygen 

 concentration falls through absorption to a lower level than the 

 optimum. When Chironomus larv83 are similarly placed in 

 a suspension of Bodo, the flagellates show no special concen- 

 tration with reference to the so-called anal gills ; furthermore, 

 as this species possesses haemoglobin, it was possible to obtain 

 independent confirmation by spectroscopic observation for the 

 conclusion that these structures have no special respiratory 

 function. It will be seen later that the so-called gills of 

 lamellibranchs are to be regarded primarily as apparatus for 

 entrapping food-particles. Bounhiol (1902) brought forward 

 evidence that respiratory exchange in some Annelids falls 

 from 25 to 75 per cent, after removal of the gills ; and 

 Winterstein's (1909) observations on the effect of occluding 

 the anus in Holuthurians seem to indicate that either the 

 " respiratory " tree or the alimentary canal is responsible for 

 about fifty per cent, of the respiratory exchange which occurs 

 in these forms. 



Of localised respiratory organs among Invertebrates the 

 most fascinating arrangement is the tracheal system of Arthro- 

 pods. Though the tracheal system of insects was fully 

 described by Malpighi in 1669, it is only comparatively re- 

 cently that the respiratory significance of the tracheal apparatus 

 has been put to conclusive experimental test, initially by the 

 work of Krogh (19 15). Krogh demonstrated the respiratory 

 function of the tracheae by two methods of attack. The first 

 consisted in analysing the gaseous contents of the tracheae of 

 the limbs in grasshoppers which had remained for some time 

 in a quiescent condition, as compared with the carbon dioxide 

 and oxygen in the tracheae of individuals which had been 

 chased to exhaustion to increase their respiration. For this 

 purpose the hindmost legs were squeezed out under glycerine 



