Ciliary Mechanisms 55 



tictilarly are sensitive to such changes. Along the shore 

 of the Gulf of Mexico, near the mouths of the Missis- 

 sippi and other rivers, the water for days at a time may 

 be nearly fresh enough to drink. 



It is sometimes difficult to determine the precise effect 

 of such changes on the inhabitants of the bottom, but 

 there is one condition, frequently observed, the response 

 to which in the case of bivalves, we are now able to de- 

 scribe in detail. This condition is the periodical loading 

 of the water with mud, that occurs on all coasts. It may 

 be observed even on the Maine coast in regions where 

 there are practically no beaches or flats, and where the 

 bottoms are very generally rocky; and almost every- 

 where along the coast of the Gulf of Mexico, but espe- 

 cially near the mouth of the Mississippi, the water is 

 clear one day and fairly thick with mud the next. The 

 phenomenon is undoubtedly due to the irregular dis- 

 charge of rivers and to tide currents. 



It may readily be understood that water bearing sus- 

 pended mud or fine sand presents a serious problem to 

 the bivalve feeding on microscopic plants strained from 

 it, and if the gill apparatus that collects them were the 

 only one to deal with them, mud as well as food would 

 necessarily be carried into the digestive tract as long as 

 the shell was allowed to remain open. This ingestion of 

 mud or sand normally occurs in some animals that are 

 able to digest the organic matter included in the mass, 

 but. with the exception of one known genus, the digestive 

 organs of shell-fish are evidently not suited to perform 

 such a function. There are two ways in which the prob- 

 lem may be solved. While the water is muddy, the 

 shell may be closed so as entirely to prevent it from 

 entering the body, or the flow may be allowed to con- 



