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- 

 nae into the slit, and was 

 held fast during a whole 

 day, being unable to with- 

 draw it. On the outer 

 face of the valve near its 

 margin is a row of gland- 

 ular 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 

 out of the passageway ani- 

 mals too big to pass 



Fig. 174. Diagram of the mechanism 

 of a trap of one of the common blad- 

 derworts. A, The trap from the 

 ventral side, showing the outspread through it — SUChaS might 

 leader hairs converging to the entrance, blockade the entrance. 

 1. leaders, r. rim, v, valve. B, A 

 median section of the same r, rim ; v, 

 valve; iv, x, y, z, epidermal hairs; 

 w, from the inner side of the rim; x, 

 from the free edge of the valve; y, 

 from the base of the valve; z, from 

 the general inner surface of the trap. decompose(L 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 



Small animals when en- 

 trapped swim about for a 

 long time inside, but in 

 the end thev die and are 



