BIOLOGICAL EFFECTS OF THE TIDES 255 



determined by the tides. Reproduction in this form is also a 

 definitely periodic phenomenon. Removed to the laboratory 

 and placed in tall test-tubes half filled with sand and half with 

 water, the animals continue to move up and down at intervals 

 strictly corresponding with particular states of the tide outside. 

 Now, in order to translate the phenomenon of tidal perio- 

 dicity into terms of life in general, one is compelled to " hedge " 

 a little. It is not altogether intended to suggest, as was done 

 for the phenomenon of stereotropism, that all the many rhythms 

 we see around us (apart from those, of course, obviously refer- 

 able to external sources) are to be explained as an echo of the 

 times when life was hammered into shape on the seashore 

 (although in some cases this is not impossible). But they are 

 almost certainly merely another form of the same thing. Thus, 

 we find ourselves very frequently employing the analogy of 

 the tides in describing, to give an example, not a few periodic 

 human functions and activities. We will go no further at 

 present than to say that the similarity is perhaps more funda- 

 mental than we are aware of. 



So far, our discussion has been of a distinctly speculative 

 character ; we may now leave speculation for facts, and give 

 our attention to the biological effects of the tides as they may 

 be observed every day upon the seashore. 



The influence of wave-impact on the life of the seashore is 

 at once made apparent to the observer, not only by the large 

 number of fixed forms, of which the rock-barnacle is a promi- 

 nent example, but also by the tendency even among free animals 

 to keep a grip of the substratum. The adhesion of barnacles 

 is due to a cement substance secreted by special glands in the 

 region of the head, but in the case of sea-anemones, flat-worms, 

 sea-slugs, and sea-snails, adhesion is due merely to an exceed- 

 ingly close contact of the body with the substratum. The 

 sticking power of the limpet is, of course, proverbial ; it is 

 thought to be due, not to the secretion of a cement substance, 

 but to the exceedingly close contact of the foot with the 

 substratum, the sole of the foot following every little irregu- 

 larity of the rock surface, and being, so to speak, rolled out 

 on the rock.* A very interesting adaptation to life in the area 

 of wave-action is seen in a fish called the lump-sucker, which 

 has the pelvic fins converted into a cup, or sucker, with which 

 it clings to rocks and weeds. Mere adhesive powers are, ap- 

 parently, not sufficient, since there is a tendency also to adopt 

 a form which offers the least possible amount of resistance to 

 the water, and, in particular, to reduce height. Shore forms 

 are typically flattened forms. It is not difficult to convince 



1 Davis, J. R. A., and Fleure, H. J., L.M.B.C. Memoir X. Patella. 

 London, 1903. 



