— 18 — 



blood — to tbe outer air. It results that the water of respiration becomes distributed tbrough the 

 mass of air in tbe tree and tbiis prevents a dessiccating action of tbe air upon tbe blood. Tbiis 

 while it migbt bappen tbat if ordinary dry atmospberic air were brougbt into relation witb tbe 

 respiratory surface of tbe blood cavity it would be fatal to tbe respiratory process, yet it remains 

 true tbat tbe onter gills are capable of functioning wben tbe animals are surrouudet by tbe ordinary 

 atmospberic medium. 



As supporting tbis view we bave tbe fact tbat tbe animals will live for a considerable lengtb 

 of time under ordinary atmospberic couditions. I bave found tbat PorceMio scaber lives from 24 to 

 36 bours wben kept in air undei- tbe conditions of an ordinary living-room. I infer tbat under 

 tbese circumstances tbe breatbing process takes place mainly in tbe outer gills and tbat finally deatb 

 ensues tbrougb tbe dessiccation of tbe inner gills only. (See also below under pbysiology of tbe 

 inner gills.) 



Conclusions. Fi'om tbe foregoing facts and considerations I reacb tbe foUowing con- 

 clusions in regard to tbe natiu'e of tbe outer brancbes of tbe first and second pairs of abdominal 

 appendages in Porcellio scaber. 



1. From tbe stand-point of morpbology tbey are gills, bomologous witli the true gills of 

 aquatic isopods. 



2. But from tbe stand-i^oint of pbysiology tbey are organs for tbe respiration of air. 



3. Tbe special respiratory organ forming tbe tree is in morpbological principle bomologous 

 witb tbe tracbeae of insects but it is different in structural plan, especially as baving only a single 

 extemal opening and as lacking spiral folds of tbe cbitinous wall. 



4. In its general pbysiological value tbe tree corresponds to tracbeae, but it differs from 

 tracbeae in tbe respects first, tbat it is adapted to retaining for a time tbe water of respii'ation in 

 tbe air-cavity and second, tbat it is an organ lying in a particular region of tbe body to wbicb tbe 

 blood is brougbt, to be aerated, instead of a system of tubes penetrating tbe body in order to carry 

 air to tbe blood. 



5. In tbe respect last mentioned tbe tree is analogous to tbe lungs of vertebrate ani- 

 mals, but 



G. Moi-pbologically tbe tree differs from lungs, fii'st, in tbat it lacks any associated musculai' 

 mecbanism for forcing air in and out and, second, it is an iufold of tbe external wall of tbe body, 

 instead of tbe wall of tbe enteric cavity. 



Tbe last tbree pairs of outer gills. Tbese lack tbe resj^iratory tree and tbe area of 

 grooved cbitinous wall. Tbat is to say, tbey bave no special modifications adapting tbem to tbe 

 res^jiration of air. In otber respects tbey correspond in structure to tbe first and second pairs. 



As regards tbeir pbysiological office it is evident from tbeir form and position tbat tbey 

 serve as covers for tbe corresponding tbree pairs of inner gills. And since tbe inner gills can 

 perform tbeir function only in air cbarged witb moisture (see below) it would appear tbat tbe outer 

 gills exercise tbeir protective function mainly in sbeltering tbe inner gills from tbe dessiccation tbat 

 would follow from direct exposure to air. 



Tbe movements of tbe outer gills bave already been referred to (p. 5). It is evident tbat 

 tbe object of tbe movements in to afiford a renewal of tbe supply of air to tbe inner gills. In a 

 living specimen under a lens one can see tbe gills separate and a layer of air enter between eacb 

 lateral division and the one tbat lies next posterior to it. As tbe gills come togetber again, tbis 



