Botany and Zoology. -281 
posterior portion projecting, for the purpose of giving ingress and 
exit to the respiratory currents of water. nsitiveness of the 
Unios were found numerously occupying the position referred to, 
plying their currents industriously through their distended orifices, 
piring in the sha ee 
he question then arose in my mind as to the possibility that 
the parts were sensitive alone to the rays of heat from the sun 
: it, by passing a scre ) 
downward, and again from below upward, it responded by closing 
its orifices as quickly as its fellows had done when my shadow 
_ Passed over them in the broad open space of sunlight. a 
Upon the supposition that the light and heat-rays are divided 
at the surface of the water, as before suggested, the heat-rays 
must all, or very nearly all, have passed down below the Unio, by 
the action of the current, while the light-rays alone reached it, 
elr sudden interception caused it to close its o us in 
