208 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



Sedentary animals, not properly swimmers, are either fixed 

 to their dwelling-place or only temporarily attached. The way 

 in which these animals either yield to or overcome the effects 

 of currents has already been indicated ; the hardness and shape 

 of the shells, the adhering power of the foot, or of the skeleton- 

 as in corals protect them against the steady latei-al pressure of 

 currents, but if they lose their hold or are broken off, they 

 are soon destroyed. Only certain creatures, as many sponges 

 and some polyps (Hydroida), although they are fixed, escape by 

 other means the destructive effects of strong currents ; their 

 extreme tenacity of texture, elasticity, and pliancy, in which 



FIG. 60. Creeping Mollusca. a, Natica, which can extend its foot very widely; b, 

 Erycina (?), which creeps by means of its foot, like a univalve, on the skin of a 

 Synapta. 



they resemble many water plants, qualify them to live even in 

 strongly agitated currents. 



Creeping creatures, which attach themselves only tempo- 

 rarily to stones and plants, and can quit them at pleasui-e, have 

 special organs of attachment. Thus, in most univalves and 

 some bivalves, the part known as the foot serves this purpose ; 

 the broad under-surface clings closely to the object they adhere 

 to, but at the same time can leave go of its hold. The force, 

 however, with which the foot adheres is not merely not absolute, 

 but is even relatively different in individuals of the same 

 species at various ages. In the course of my experiments on 

 the pond-snail (Lymncea staynalis) I found that those just come 



