THE BREATHING APPARATUS 297 
city of the membrane, and the expelled water is thus 
prevented from returning. 
On the other hand, a new 
supply of water can now 
enter through the openings 
at the sides of the maxille, 
and these openings in their 
turn are then closed by the 
forward movement of the 
gill-plates. By this means 
the gills are kept washed 
by a continually fresh cur- 
rent of water, of essential 
service to the health and 
vital energy of the animal, 
although it is obvious that 
the creature when burying 
itself in the sand can for a 
time dispense with this mode 
of aerating its blood. In 
the Epicaridea, a group of 
Isopoda, to be mentioned 
hereafter, the marsupial 
plates of the first gnatho- 
pods (second maxillipeds) 
have a function similar to 
that of the epipods just de- 
scribed. 
In the egg-bearing fe- 
males of the Cumacea, the 
second pair of maxillipeds 
have at their base a broad 
fan directed straight back- 
wards within the ventral 
body wall. This is formed 
by the two epipods, each 
represented by a small round 
plate armed on its free mMar- Fic. 24.—Diastylis stugia, Sars. First max- 
3 : illiped, with branchial apparatus, female 
gin with several long sete. — specimen [Sars]. 
