ANIMAL LIFE OF THE SHORES 297 



parts of the mantle-lobes, i.e. the parts near the edge 

 of the shell, are much thickened and corrugated ; the 

 thickenings help to ^seal up the mantle-cavity and the 

 corrugations probably contain a certain amount of water, 

 so that the mantle-cavity is always moist, and evaporation 

 is reduced to a minimum. In fact, the tightly fitting 

 junction between the upper or left valve and the palm 

 stem is supplemented by an internal thick cushion which 

 acts like the indiarubber washer of a watertight joint in 

 a metal pipe. The mantle-cavity is also produced here 

 and there into sinuses which run into the body, and they 

 can be cut off from the main cavity by ridged folds with 

 interlocking hairs. Professor Bourne considers these 

 extensions of the mantle-cavity to be water reservoirs. 

 One other feature of this Mollusc's anatomy is worthy 

 of attention. There occurs on the left or upper mantle- 

 lobe a ring of pigmented spots ; these are rudimentary 

 eyes with cornea, lens, and retina, or at least primitive 

 representatives of these structures. As they are situated 

 at some distance from the edge of the mantle-lobe, it 

 is obvious that light can only reach them through the 

 upper valve of the shell, and since this shell is translucent, 

 but by no means perfectly transparent, the light that 

 reaches the eye-spots must be very dim. The structure 

 of the eye-spots is so simple that probably the utmost 

 extent of their powers is to discriminate between dark- 

 ness and light, and Professor Bourne suggests that they 

 are of use in giving warning to the animal to keep the 

 valves of the shell closely pressed to the palm stem during 

 the heat of the day. 



Another denizen of the mangrove-swamps at Santu- 

 bong was a little crab of the genus Sesarma, bright blue 

 in colour. At low tide it could be seen running about 



