24 
to me to warrant the inference that it functions directly as an organ of respiration of air. It is 
true that as the blood circulates through the gill it is separated from the external atmosphere hy 
the rather thick ventral wall of chitine. But through the movements of the gill, taking place as 
described for PorceJUo air is constantly brought into relation with the inner surface where the 
chitinous wall is thin. It seems j:)robahle that the conditions re(piisite for an exchange of gases 
between the blood and the air are hei’e satisfied. 
In addition to the resi^iratoiy function the outer gills serve as covers for tlie innei' gills. 
The Inner Gills. The last three pairs of inner gills of Ligidium ai’e in all respects 
identical in general structure and relations with those of Porcellio scaher. The first two pairs, as 
already stated, p. 6, are rudimentary. 
Oniscus murarius Cuvier. 
In the genus Oniscus the anatomy of the outer gills differs greatly from that of the genera 
thus far described. They lack any trachea-like structures but in their jolace possess special modi- 
fications adapted to the same functional end. 
The first right outer gill of the male, examined on the inner or dorsal surface is reju'esented 
in Fig. 11. It is seen that the gill is somewhat triangular in general outline, with rounded corners. 
The articulation of the gill with the basal joint (art) is situated on the aiiterior and outer side. 
The inner side lies parallel with the middle line of the l)ody and is nearly straight. The side 
lying outwardly and posteriorly is deeply indented, the angle thus formed dividing this portion of 
the gill into two unequal lol)es. The outer of these lobes is the portion of the gill specially modified 
in adaptation to the resi^iration of air. Viewed from above it is seen to constitute the outer- 
posterior portion of the gill and to comprise about one-third of the whole surface. Its margin is 
semi-circular in shape and its line of union with the general part of the gill is slightly convex 
toward that part. Along this line is situated a broad cliannel which, as will he shown below, con- 
ducts blood, after being aerated in the gill, hack to the body and may be designated the main 
excurrent channel. Communicating with this channel is a series of small channels, arranged in a 
somewhat radiate manner, and occupying the whole area of the special part of the gill. Another 
broad channel is situated along the anterior and inner margin of the gill. As will he shown below, 
this receives blood from the body and it may be designated the main incurrent channel. 
The bonndaries of all the blood-channels ai’e marked by rows of dots. Such dots are 
also seen scattered over the middle region of the gill, more or less aggregated into groups. 
The chitinous covering of the gill is seen as a somewhat broad line along the entire margin 
of the gill. It is provided with stiff hairs on the outer and posterior border. 
If the gill of a living animal is examined with a hand lens the special part presents the 
appearance of containing air. This may be better seen by placing a specimen in water under a 
cover-glass and examining with a low power of the microscope. Corresponding with the radiate 
channels above noted are seen a series of wave-like elevations and depressions which under reflected 
light appear silver-white and glistening. Most of the waves divide toward the margin of the gill, 
becoming Y-shaped, thus giving rise to the radiate arrangement of the parts, as a whole. Under- 
neath the wave-like elevations the corpuscles of the blood may he seen to circulate. 
