146 THE DISTRIBUTION OF THE PLANKTON [PART II 



potent cause of the vertical irregularity of the plankton, and some 

 proof of this may be given. If a tow-net is lowered to the bottom 

 of the Irish Sea during the day a catch is obtained which nearly 

 always contains Sagitta though this worm may be absent from the 

 catch made at the surface at the same time. So also if a catch is 

 made at the surface during the night this almost invariably con- 

 tains numerous specimens of the same animal. Both at the sea 

 bottom during the day and at the surface during the night, light 

 is absent, or nearly so. If again we empty a plankton catch into 

 a glass " crystallising dish " and stand this at a window we will 

 always find that there will soon be a distinct segregation of the 

 copepods : some of these will swim over to the side of the dish 

 nearest to the source of light, while others will seek the darker 

 parts of the vessel. The same behaviour of copepods towards 

 light can often be observed when we put a catch of these crus- 

 taceans into a w^atch glass and place this on the stage of a 

 microscope. If all light is then excluded from the catch, except 

 that which comes up from beneath through a narrow^ aperture in 

 a diaphragm, w^e may find that the copepods will crowd round 

 this pencil of light. But heliotropism is even more beautifully 

 shewn by the larvae of the nereid worm Phyllodoce. This animal 

 deposits its eggs in albuminous cocoons w^hich are laid on the 

 shore and if one of these structures, containing motile trochophores, 

 be cut open in a glass dish containing sea w^ater the larvae will at 

 once swim across to the light and form a dense layer on the sides 

 of the glass nearest to the source of light. 



Probably the majority of the organisms forming the plankton 

 are affected to some extent by changes in the intensity of light, 

 and although no very systematic or exhaustive observations have 

 been made on this subject it is nevertheless the case that consider- 

 able vertical migrations are the results of this cause. We should 

 expect to find, and indeed do find, that there is a diurnal vertical 

 movement of the plankton accompanying the alternations of day and 

 night, and possibly there is also a seasonal variation in the nature and 

 abundance of the surface plankton as the result of the yearly varia- 

 tion in the amount of light which falls on the surface of the sea. 

 Finally one may conjecture that the difference between the 

 plankton at the surface on a bright day, and that obtained on a 



