0 
They lie in the outer and posterior portion of the plates and the posterior margins of the same 
plates present indentations. These mark the places where small huhhles of gas pass out from the 
interior of the corpus alhum when a gill is placed under Avater and pressure ap})lied. The use of 
the microscope is requisite for this demonstration. 
In the genus Oniscus the outer branches of all the abdominal appendages (the gill-covers 
of authors) present also definite modifications but morphologically entirely different from those of 
ForcelUo and Armadillidiimi. The outer and posterior portion of each gill is thinner than the rest 
and is marked by lines arranged in a I'adiate manner. To tlie eye these lines ap}>ear as glistening 
Avhitish spaces, suggesting the presence of chamber^ containing air. When an animal is placed 
under water these spaces gradually disappear; when the same animal is returned to air the original 
appearance is gradually regained. 
The inner divisions of the abdominal appendages are, excepting in Ligidium, uniform in 
structure and relations for all genera which I have examined. They consist of three pairs of deli- 
cate flattened sacks attached to the third, fourth and fifth abdominal segments. They lie within 
the corresponding outer divisions and share with them movements through the muscles of the com- 
mon basal joint. 
These organs have been designated as gills by all authors. 
In Ligidium all five of the inner branches of the appendages are re])resented. Those of 
the first pair consist of thin plates resembling in general the outer divisions. The inner branches 
of the second pair are rudimentary gills. The other three pairs are identical in general features 
Avith the corresponding pai'ts in the other genera. 
