20 
continued existence is evident from tlieir habits of life and is readily susceptible of demonstration 
by experiment. That it is the inner rather than the outer gills whose function is conditioned by 
the presence of moisture would seem evident from the following considerations, as well as from 
those given above in the account of the physiology of the outer gills: 
The inner gills are protected from direct exposure to the surrounding atmospheric air, while 
the outer gills function under such exposime. 
Those genera of land-isopods which lack special respiratory structures in the outer gills, 
such as Ligidiiim, live only in very moist situations. 
Considerations as regards the phylogenetic history of the land-isopods ; As the ancestral 
aquatic forms gradually acquired the habit of terrestrial life, the outer gills underwent far greater 
modification than the inner gills. In all they became converted into covers for the inner ones but 
in this transformation probably did not entirely lose their functional capacity as gills. In certain 
genera, as Ligidimn, the process of modification stopped at this point. In other genera, as Por- 
cellio, a new form of modification set in, namely, that of the development of tracheae-like structures. 
This was in adaptation to the respiration of ordinary atmospheric air. But the inner gills acquired 
only a certain measure of adaptation to respiration in a medium of air; hence the presence of 
moisture in the air surrounding them is still *indis2)ensable to their functional action. 
Porcellio Batzhurgliil Beandt. 
In this species all five j^airs of the outer gills j)0ssess the special respiratory structures 
described for Porcellio scaher. They are best developed in tlie first i^air and gradually decrease in 
size to the last pair. I find that in all essential structural features the outer gills are identical 
in the two species. The respiratory tree and grooves present the same characters in both, dift’ering 
only as regards the minor features of size and form. 
It is worthy of remark, as bearing upon the question of the function of the outer gills, 
that this species lives in situations where the air is charged with moisture only in a moderate degree 
in excess of that of ordinary atmospheric air. Their habitat is under the bark of dead trees and 
they may often he found a meter or more above the ground. 
Cylistictis convexicus Budde-Lund. 
This is the species which under the name of Porcellio armadilloides was investigated by 
Leydig.*. I have established the identity as to species of the animals studied by Leydig and those 
studied by myself not only by reference to the list of synonyms given in Budde-Lund’s work hut 
also by a comparison of preparations of the gills with the figures of the same given by Leydig. 
In this species all five pairs of the outer gills have the special respiratory structures — 
the tree and the grooves. I have carefully examined, both as seen from without and in section. 
fiber Ampbipoda und Isopoda ; Zeitscbrift fiir wissenschaftliclie Zoologie, Bd. 30, Suppl. 1878. 
