REPRODUCTION AND GROWTH 247 



Shore Forms with Pelagic Larvae. — A question which 

 can hardly fail to exercise the minds of those studying the 

 reproduction of shore animals is, how does it come about 

 that while, apparently, pelagic larvae are continually being 

 driven and dispersed over vast areas, the adult forms are 

 invariably restricted to constant and definite portions of 

 the shore ? According to Lo Bianco (1909), the answer to 

 this question lies in the way in which the reactions of the 

 larvae towards the light are related to the periodic circulation 

 of coastal water and in particular to those coastwise currents 

 which, owing to the wealth of pelagic forms they contain, 

 are known as zoo-currents. 



During the day, the larvae of such typically shore forms 

 as Sycon, Balanus, Echinus, Ascidea, Amphtoxus, etc., owing 

 to their strong positive phototropism, occupy the surface 

 layers of the sea (" phaoplanktonic ") ; whereas at night, owing 

 to the absence of light stimulus, they may be found occu- 

 pying a zone extending from the surface to a depth of 

 several metres ('* knephoplanktonic "). Now, as is well known, 

 owing to the unequal heating of land and sea by the sun, 

 the general direction of the wind during the day is on-shore 

 (sea-breeze), while at night it is in the opposite direction 

 (land-breeze). Consequently, during the day, the larvae 

 which occur at the surface are driven towards the coast 

 where they are caught in the littoral currents already referred 

 to. On completing their metamorphosis the young animals 

 drop to the bottom and, being still positively phototropic, 

 are guided to their final habitat in the tidal area. The 

 difference in the degree of sensitiveness to light determines 

 why, for instance, a form like Arbacia pustulosa finds its 

 final habitat among the rocks in a few feet of water, while 

 Echinus microtuherculatus is led towards the deeper water and 

 its typical habitat among the " meadows " of Posidonia. 



Generally speaking, larval forms which are negatively 

 phototropic undergo their development in the middle waters 

 or knephoplanktonic zone, while positively phototropic 

 forms develop in the surface waters or phaoplanktonic zone. 



It is thus evident that the survival of shore forms with 



