THE ECHINODERMATA 12$ 



stricted, forming merely a rosette at the top of 

 the shell : hence they are described as circum- 

 script or " petaloid." The excretory aperture is 

 shown in the photograph as a smaller dot on one 

 side of the mouth, while in the Echinus, on the 

 contrary, it is at the top of the shell. The five 

 odd-looking, elongated holes are a curious indi- 



FlG. 36. The Five-holed Sand Cake, Mellita pentapora, a flat 

 sea-urchin from the east coast of tropical North America. A, 

 upper surface ; B, lower surface ; C, side view. 



vidual peculiarity of this Sea-Urchin. It has 

 already been explained that the Shield-Urchins 

 are flattened in order to distribute their weight; 

 these holes are a contrivance for still further 

 reducing the weight in comparison with the area. 

 This is when the animal is lying quiet at the bot- 

 tom of the water, but when it moves about what 

 effect will the presence of the holes produce ? 

 Flattened animals are usually supposed to derive 

 an advantage from the fact that they sink more 

 slowly through depths of water; as in lying upon 

 the ground, their weight is distributed, and they 



