252 SCIENCE PROGRESS 



In estimating the importance of the tides to life, whether 

 to life in general or to the present life of the sea-shore in par- 

 ticular, we require to give separate consideration to at least 

 three distinct ways in which the tides act. These are : (i) by 

 causing successive strips of the sea-shore to be pounded more 

 or less violently ; in other words, by impact ; (2) by subjecting 

 the area exposed to their action to alternate hydration and 

 desiccation ; (3) by leaving a rhythmical impress upon organisms 

 subject to their influence. Each of these modes of action may 

 now be taken in turn and examined in its general biological 

 relation. How, for instance, do we explain the origin of a 

 sedentary habit in animals ? We can understand the sedentary 

 habit in land-plants, of course, but why in marine plants (where 

 the food substances can be absorbed by the whole surface of 

 the plant directly from the water), and, still more, why in 

 marine animals ? The earliest forms of life were almost cer- 

 tainly not fixed forms. The great number of sedentary animals 

 on (the sea-shore, sponges, hydroids, polyzoa, barnacles, sea- 

 squirts, and so on, and the obvious value of some form of 

 anchorage in this particular area seem to make it almost certain 

 that the stimulus prompting the sedentary habit in animals is 

 wave-impact, just as no botanist would have any hesitation in 

 ascribing the fixed habit of the larger algae to the same cause. 



Following up this question of wave-impact a little further, 

 we meet another with striking phenomenon which, it seems 

 to us, may not impossibly be correlated with tidal conditions. 

 This phenomenon, which is of very wide-spread occurrence, is 

 known as stereotropism, or stereotaxis (according to whether 

 the animal which exhibits it is fixed or freely moving). The 

 term ** stereotropism " expresses the tendency a great many 

 animals have to react in very marked fashion to stimuli of 

 contact, and is well illustrated by a case which the writer him- 

 self has studied. On nearly all sandy-muddy shores, where there 

 are stones embedded in the substratum, there occurs, often in 

 considerable numbers, a worm which is known as Cirratulus.^ 

 It is not unlike an earthworm in appearance, but has sprouting 

 from each segment a pair of delicate, rosy processes which 

 look like tentacles, together with a bunch of such processes 

 behind the head. It is always found lying in such a way as 

 to bring every portion of one surface of its body into contact 

 with a stone. If one of these worms is placed in a vessel of 

 sea-water devoid of stones the animal ties itself literally into 

 knots and appears to be very ill at ease. Place a large flat 

 stone in the vessel, and the worm, immediately on coming into 

 contact with it, begins to burrow beneath it. A few minutes 



^ Flattely, F. W., Ecology of Cirratulus tentaculatus, Journ. Mar. Biol, 

 Assoc. > xi, 1916. 



