570 RESPIRATORY EXCHANGE IN AQUATIC ORGANISMS 



These larvae have on the ventral surface of the eighth abdominal 

 segment four tube-like hollow outgrowths, in which the blood cir- 

 culates. On the dorsal surface of the last abdominal segment there 

 are four shorter hollow outgrowths, also with a blood stream through 

 them. All these processes are thin-walled and have a thin cuticle. 

 It is usually assumed that the chief respiratory gas-exchange takes 

 place through the walls of these outgrowths, the so-called ventral and 

 anal gills. When the larvae are tested with the Bodo suspension, it 

 is found that oxygen is absorbed by the whole of the general body 

 surface of the larva except the head which has a very thick cuticle. 

 The intensity of absorption is usually greatest in the posterior abdom- 

 inal segments, but this is not invariably so. The most actively 

 respiring segments vary from one individual to another, and when a 

 single individual is tested a number of successive times the most 

 actively respiring regions may vary in position each time. However, 

 the remarkable result is the behavior of the so-called gills. No more 

 oxygen is found to be absorbed by the "anal gills" than by the general 

 body-surface and no oxygen at all is absorbed by the "ventral gills." 

 The latter may be seen to project through and beyond the collection 

 or band of flagellates without influencing it (Figs. 2 and 3). 



This result can be confirmed by quite another mode of experimen- 

 tation. The blood of these larvae contains hemoglobin dissolved in 

 it. When a larva is examined with a microspectroscope the whole 

 body shows the oxyhemoglobin absorption bands. If a larva is now 

 placed in water in a hollow-ground slide and covered with a cover- 

 slip, the edges of which are sealed down with vaseline, at the end of 

 about 20 minutes the whole body of the larva shows the reduced 

 hemoglobin absorption spectrum. The larva does not die when this 

 has occurred. It will remain alive with heart beating for many 

 hours beneath the sealed cover-slip. If a small bubble of air is now 

 allowed to enter beneath the cover-slip, so that it comes to rest 

 up against the surface of the larva, an examination with the micro- 

 spectroscope shows that oxyhemoglobin first appears inside the body 

 of the larva at a point nearest to the bubble and thence it spreads 

 over the rest of the body. Thus the part of the body-surface nearest to 

 the bubble first absorbs the oxygen, and not the "gills." But a more 

 striking result than this can be obtained. If by chance the bubble 



