— 24 — 



to me to Warrant tlie inference tliat it functions dii-ectly as an organ of i'espiration of air. It is 

 true that as the blood circulates throiigh the gill it is separated from tlie external atmosphere by 

 the rather tbick A'entral wall of cliitine. But tbrough tbe movements of the gill, taking place as 

 described for Porcellio air is constantly brought into relation witli the inner siirface where tbe 

 cbitinous wall is thin. It seems probable that tbe conditions re(iuisite for an excbange of gases 

 between the blood and the air are here satistied. 



In addition to tbe respiratory funetion the outer gills serve as Covers for tbe inner gills. 



Tbe Inner Gills. The last three pairs of inner gills of Ligidmm are in all respects 

 identical in general structure and relations with tbose of Porcellio scaber. Tbe first two pairs, as 

 already stated, p. 6, are rudimentary. 



Oniscus murarius Cuvier. 



In tbe genus Oniscus the anatomy of the outer gills differs greatly from that of tbe genera 

 thus far described. Tbey lack any tracbea-like structures but in their place possess special modi- 

 fications adapted to tbe same functional end. 



The first right outer gill of tbe male, examined on the inner or dorsal surface is reprcsented 

 in Fig. 11. It is seen that the gill is somewbat triangulär in general outline, with rounded corners. 

 The articulation of the gill with tbe basal Joint (art) is situated on tbe anterior and outer side. 

 The inner side lies parallel with the middle line of tb^- liody and is nearly straight. The side 

 lying outwardly and posteriorly is deeply indented, tbe angle thus formed dividing tbis poition of 

 the gill into two unequal lobes. Tbe outer of tbese lobes is the portion of the gill specially modified 

 in adaptation to the respiration of air. Viewed from above it is seen to constitute tbe outer- 

 posterior portion of the gill and to coniprise about one-tbird of tbe whole surface. Its margin is 

 semi-circular in shape and its line of union with tbe general part of tbe gill is slightly convex 

 toward that part. Along tbis line is situated a broad Channel which, as will be shown below, con- 

 ducts l)lood, after being aerated in the gill, ])ack to the body and may be designated tbe maiu 

 excurrent cbannel. Communicating with tbis cbannel is a series of small Channels, arranged in a 

 somewbat radiate mauner, and occupying the whole area of the special pai't of the gill. Anotber 

 broad cbannel is situated along tbe anterior and inner margin of tbe gill. As will be shown below, 

 tbis receives blood from the body and it may be designated the main incurrent cbannel. 



The bonndaries of all tbe blood-cbannels are marked by rows of dots. Such dots are 

 also seen scattered over tbe middle region of the gill, more or less aggregated into groups. 



The cbitinous covering of tbe gill is seen as a somewbat broad line along the entire margin 

 of tbe gill. It is provided with stitf bairs on the outer and posterior border. 



If tbe gill of a living animal is examined with a band lens tbe special part presents the 

 appearance of containing air-. Tbis may be better seen by placing a specimen in water under a 

 cover-glass and examining with a low power of tbe microscope. Corresponding with tbe radiate 

 Channels above noted are seen a series of wave-like elevations and depressious which under reflected 

 light appear silver-white and glistening. Most of the waves divid(^ toward the margin of the gill, 

 becoming Y-sbaped, thus giving rise to the radiate arrangement of the parts, as a whole. Under- 

 neath tbe wave-like elevations the corpuscles of tbe blood may be seen to circulate. • 



