286 Adjustment to Conditions of Aquatic Life 



one side so that it will readily push inward, but holding 

 tightly against the rim when pressed outward. This 



is the most important 

 single feature of the trap. 

 It makes possible getting 

 in easily and impossible 

 getting out at all. Dar- 

 win speaks of a Daphnia 

 which inserted an anten- 

 nse into the slit, and was 

 held fast during a whole 

 day, being imable to with- 

 draw it. On the outer 

 face of the valve near its 

 margin is a row of glan- 

 dular hairs. These have 

 roundly swollen terminal 

 secreting cells. They may 

 be alluring in function, tho 

 this has not been proven. 

 Directed backward across 

 the center of the valve are 

 four stiff bristles, that 

 may be useful for keeping 



Fig. 174. Diagram of the mechanism OUt of thepaSSagCWay ani- 



of atrapof oneof thecommonblad- j;nals tOO big tO paSS 

 derworts. A, The trap from the . . °. K ^ 



ventral side, showing the outspread through it SUCh aS might 



leader hairs converging to the entrance blockade the entrance. 

 1. leaders, r. nm, v, valve. B, A ^ ,, . , . 

 median section of the same r, rim; v, bmall animalS when en- 

 valve; w x,y, 3, epidermal hairs; trapped swim about for a 



■w, Irom the mner side of the nm; x, . , ■ ■ ■ ■, , , • 



from the free edge of the valve; y, long time mSldc, but m 



from the base of the valve; 2, from ^he end they die and are 



the general mner surface of the trap. , 1 ^t ^ 



decomposed. New traps 

 are of a bright translucent greenish color; old ones are 

 blackish from the animal remains they contain. The 

 inner surface of the trap is almost completely covered 



