— 20 — 



continued existence is evident from their habits of life and is readily susceptible of demonsti'ation 

 by experiment. That it is the inner rather than the outer gills whose function is conditioned by 

 the presence of nioisture would seern evident from the following considerations, as well as from 

 those given above in the account of tlie i^hysiology of tlie outer gills: 



The inner gills are protected from direct exposure to the surrounding atmospheric air, while 

 the outer gills function under such exposm-e. 



Those genera of land-isopods which lack special respiratory structures in the outer gills, 

 such as Ligidium, live only in very meist 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 fuuctional capacity as gills. In certain 

 genera, as Ligidium, the process of modification stopped at this point. In other genera, as For- 

 cellio, a new form of modification set in, namely, that of tlie development of tracheae-like structures. 

 This was in adaptation to the respiratiou of ordinary atmospheric air. But the inner gills acquired 

 only a certain measure of adaptation to respiratiou in a medium of air; hence the presence of 

 moisture in the air surrounding them is still indispensable to their functional action. 



Porcellio Ratzhurghii Brandt. 



In this species all tive pairs of the outer gills possess the special respiratory structures 

 described for Porcellio scaber. They are best developed in the first pair and gradually decrease in 

 size to the last pair. I find that in all essential structural features the outer giUs are identical 

 in the two species. The respiratory tree and grooves present the same characters in both, differing 

 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 hark of dead trees and 

 they may offen be found a meter or more above the ground. 



Cylisticus convexicus Budde-Lund. 



This is the species which under the name of Porcellio armadilloides was investigated by 

 Leydig.' I have estaldishcd 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 but 

 also by a comparison of preparations of the gills with the figures of the same given by Leydk;. 



In this species all five pairs of the outer gills have tlu' si)ecial respiratory structures — 

 the tree and the grooves. I have carefuUy examined, lioth as seen from without and in section. 



Über Amiihipoda luid Isopoda; Zeitschrift für wissenschaftliche Zoologie, Bd. 30, Öuppl. 1878. 



