REACTIONS OF LAND ISOPODS TO LIGHT 233 



always been the center of interest in these animals, and recent 

 studies have been made by StoUer ('99), Bepler ('09), Bernecker 

 ('09), Unwin ('09), Muller ('10), and Herold ('13). The fol- 

 lowing statements represent perhaps the most widely accepted 

 opinion among these investigators. 



1. In most and probably all genera the thin-walled inner 

 branches (endopodites) of the pleopods are used for respiration. 

 They are covered and thus protected by the outer branches 

 (exopodites) and are kept moist by glandular secretions. 



2. The outer branches in most forms, besides serving as a pro- 

 tection for the inner branches, have trachea-like modifications 

 for breathing atmospheric air. Of the two genera used in the 

 experiments reported in the present paper, Porcellio has these 

 structures better developed than Oniscus. Correlated with this 

 fact is the abihty of Porcellio to live in drier situations than 

 Oniscus. Of the species of Porcellio, P. rathkei is somewhat 

 better adapted to air-breathing than P. scaber. 



3. The character of the endopodites resembles closely that of 

 the same structures in Asellus communis, the fresh-water isopod 

 previously referred to. The most primitive land isopods, the 

 family Ligydidae, have ' gills' closely resembling those of Asellus. 

 This seems to indicate that isopods as a group were particularly 

 suited to a change from water to land, since only slight modi- 

 fications in the respiratory mechanism proved necessary. 



b. Food. The food and feeding habits of land isopods and of 

 Asellus have been investigated, respectively, by Murlin ('02) and 

 Banta ('10, p. 477). Both kinds of animals exercise little selec- 

 tion in their choice of food and take a large percentage of inor- 

 ganic matter into the digestive tract with the food. 



c. Care of young. The developing young of isopods remain 

 in a brood-pouch on the ventral surface of the female until they 

 have developed into the adult structure. In this way the young 

 are protected during their early existence and no special device 

 during the early stages is necessary to adapt them to land life. 



d. Behavior. So far as respiration, feeding habits, and pro- 

 tection of developing young are concerned, land isopods are 

 little different from their aquatic relatives. The isopod plan of 



