— 29 — 



2. The preceding experiineiit was repeated, excepting that the animal was placed in normal 

 salt Solution instead of water. The result w^as that the Chambers contained blood plasma, but per- 

 haps less in (piantity than in the previous case. 



In this case the two fluids were of approximately equal densities and, it may be inferred, 

 tended to replace the air in equal measure. Allowance must be made, however, for the greater 

 thinness of the wall bounding the blood-chamber than the outer chitinous wall. 



3. The experiment was repeated using a concentrated salt Solution. The result was that 

 the Chambers appeared as empty, or nearly so, little blood plasma^ having entered them. 



In this case it would appear that the more concentrated fluid replaced, in the main, the air. 



In all three cases the results become intelligible on the supposition that the Chambers con- 

 tain air. Thus these experiments wai-rant the inference that the Chambers normally contain air 

 and not blood. 



In considering more precisely the function of the air Chambers we must first take account 

 of the fact that they do not serve to increase the area of respiratory sm-face. They are not, as 

 in the case of ForceUh and its congeners formed by an inward fold of the wall of the gill but are 

 simply Spaces hlled with air, lying betweeu the parallel walls of the blood cavity and the gill. But 

 it is to be noted that a provision for exposui'e of a large surface of blood to air is made by the 

 expandet dimensions of the special part of the gill and by the presence of these parts in all five 

 pairs of the outer gills. 



In studying the outer gills of Porcellio we reached the conclusion that the respiratory tree 

 is an adaptation having a two-fold purpose ; namely, fii-st, to provide for a large surface of exposure 

 of blood to air and, second, to seciu'e the protection of the blood against dessiccation from exposiu-e 

 to air in the process of respiration. Now, in Oniscus, the former of these pui-poses being provided 

 for by the means just noted — an increase of the total area of the special respiratory surface — 

 it is left to the air-chambers to prevent a too great loss of water from the blood in the jarocess 

 of respiration. One may easily conceive that if nothing intervened between the blood and the out- 

 side atmospheric air but the two very thin membranes of the wall of the blood cavity and the chi- 

 tinous wall of the gill, the two being in contact, dessiccation would follow. Again, phylogenetic 

 considerations lead us to expect that the respiratory sui'faces must be maintained in a moist state 

 as a condition of their functional action. 



In the respiratory process the air of the Chambers becomes charged with water of respiration. 

 The only means of escape of this water to tlie outer atmospheric air is by passage through the 

 chitinous wall. This process takes place slowly and secures the maintenance of the moist condi- 

 tion of the air in the Chambers. This air is thus separated from the blood by only a very thin 

 moist membrane. Thus conditions most favorable to respiration are provided; the blood is both 

 shielded from dessiccation and at the same time separated from air by only a very slight barrier. 



I reach the conclusion that through the possession of these special modifications, the outer 

 gills of Oniscus constitute organs for breathing ordinary dry atmospheric air. The several collateral 

 reasons in support of this conclusion that have already been given for Forcellio apply also for Oniscus. 



The Inner Gills. The inner gills of Oniscus agree in all essential featui-es with the 

 corresponding parts in Forcellio and the other genera already described. 



