92 New York State Museum 



ment of some of the gill filaments which acts as a stopper 

 for the mouth of the tube when the animal has withdrawn 

 into it. The tube serves therefore to prevent dessication 

 (drying out) as well as for protection. Burrowing is 

 one of the commonest habits used to avoid dessication as 

 well as to avoid enemies, and is used by many animals. 

 The higher the position of animals along the shore the 

 greater their capacity to resist dessication. The family 

 to which the Periwinkles belong (Littorinidae) has been 

 regarded as on the road to a terrestrial life. Certain of 

 the species live above the high-water mark in shady crev- 

 ices and thrive on the water from the dashing spray and 

 moisture present because of close p'roximity to the sea. 

 There are tropical species that have even less moisture. 

 With the capacity to resist dessication in these forms 

 goes also a capacity to endure freshening of the water. 

 This matter will be discussed under the subject of salinity. 



Effects of currents. Currents also play their part in 

 the struggle for existence among shore animals. They 

 act as an agent of dispersal for both plants and animals. 

 Many animals that are attached in the adult stage are 

 free-swimming or floating in the early growth stages and 

 are therefore for a time part of the plankton of the sea, 

 and subject to the action of currents. Many of these 

 forms, when it comes time to settle, return to some place 

 near the point of origin, but others have been carried by 

 the currents to far distant places whence there is no re- 

 turn and to which they must adapt themselves. The 

 region may be more or less favorable. If too unfavor- 

 able, they succumb and die; if the conditions are favor- 

 able enough to secure a foothold, they still are forced 

 into competition with the local species which may be more 



