THE BREATHIJ^G APPARATUS 



297 



city of tlie 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 maxillf^, 

 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 

 b}' 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 Epicaridca, 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 mar- Fig. U.—DiastyUs stvgia, Sars. First mas- 

 • , n 11 , illiped, with branchial apparatus, female 



gm with several long Set^. specimen [SarsJ. 



