18 
blood — to the outer air. It results that the water of respiration becomes distributed through the 
mass of air in the tree and thus prevents a dessiccating action of the air upon the blood. Thus 
while it might happen that if ordinary dry atmospheric air were brought into relation with the 
respiratory surface of the blood cavity it would be fatal to the respiratory process, yet it remains 
true that the outer gills are capable of functioning when the animals are surroundet by the ordinary 
atmospheric medium. 
As supporting this view we have the fact that the animals will live for a considerable length 
of time under ordinary atmospheric conditions. I have found that Porcellio scaber lives from 24 to 
36 hours when kept in air under the conditions of an ordinary living-room. I infer that under 
these circumstances the breathing process takes place mainly in the outer gills and that finally death 
ensues through the dessiccation of the inner gills only. (See also below under physiology of the 
inner gills.) 
Conclusions. From the foregoing facts and considerations I reach the following con- 
clusions in regard to the natime of the outer branches of the first and second pairs of abdominal 
appendages in Porcellio scaber. 
1. From the stand-point of morphology they are gills, homologous with the true gills of 
aquatic isopods. 
2. But from the stand-point of physiology they are organs for tlie respiration of air. 
3. The special respiratory organ forming the tree is in morphological principle homologous 
Avitli the tracheae of insects but it is different in structural plan, especially as having only a single 
external opening and as lacking spiral folds of the chitinous wall. 
4. In its general physiological value the tree corresponds to tracheae, but it differs from 
tracheae in the respects first, that it is adapted to retaining for a time the water of respiration in 
the air-cavity and second, that it is an organ lying in a particular region of the body to which the 
blood is brought, to be aerated, instead of a system of tubes penetrating the body in order to carry 
air to the blood. 
5. In the respect last mentioned the tree is analogous to the lungs of vertebrate ani- 
mals, but 
6. Morphologically the tree differs from lungs, first, in that it lacks any associated muscular 
mechanism for forcing air in and out and, second, it is an infold of the external wall of the body, 
instead of the wall of the enteric cavity. 
The last three pairs of outer gills. These lack the respiratory tree and the area of 
grooved chitinous wall. That is to say, they have no special modifications adapting them to the 
respiration of air. In other respects they correspond in structure to the first and second pau’s. 
As regards their physiological office it is evident from their form and position that they 
serve as covers for the corresponding three pairs of inner gills. And since the inner gills can 
perform their function only in air charged with moisture (see below) it would appear that the outer 
gills exercise their protective function mainly in sheltering the inner gills from the dessiccation that 
woidd follow from direct exposure to air. 
The movements of the outer gills have already been referred to (p. 5). It is eAudent that 
the object of the movements in to afford a renewal of the supply of air to the inner gills. In a 
living specimen under a lens one can see the gills sejAarate and a layer of air enter between each 
lateral division and the one that lies next posterior to it. As the gills come together again, this 
