246 H P. KJERSKOG AGERSBORG. 



two stated periods may not be effected by any one of these 

 causes, separately. For still other factors enter it, and, therefore, 

 more frequently several causes may cooperate, resulting in defi- 

 nite movements at the same time. Thus the variable conditions 

 within the animal may cause different reactions toward light at 

 various times. 



Loeb (6) has shown that the caterpillars of Porthesia clrrysor- 

 rhcea, after fasting through the winter, are energetically positively 

 heliotropic; but that after these animals have eaten, heliotropism 

 is not shown so definitely Plant lice become positively helio- 

 tropic only after they have fed; the larvae Musea vomitoria are 

 energetically negatively heliotropic only when fully grown, etc. 



Heliotropism in Pycnopodia is, however, only a partial cause 

 for migration or distance-movements, and the periods of migra- 

 tion are not very definitely marked ; they are quite extended in 

 time. Various external conditions, as well as internal, contribute 

 to this. 



Search for Food. The food problem is another factor which 

 determines the distance-movements in Pycnopodia. It will, like 

 other starfish, move according to the distribution of food. If 

 Pycnopodia finds its surroundings scarce in food material, it has 

 no difficulty in taking leave for different grounds. In fact, as 

 it has been found in bays at a depth of twenty fathoms where 

 there would be a great variety of food, e. g., sea-urchins (Strongylo- 

 centrotus drobaciencis) , Pectens, Yoldia, etc., when conditions for 

 it would not be favorable to remain on other grounds, such as in 

 shallow water during the summer, indications seem to point 

 that way. 



Breeding Impulse. Temperature together with the impulse to 

 breed are still other causes for migration. The temperature below 

 ten fathoms is comparatively constant during the entire year, while 

 the temperature at the surface varies quite considerable during the 

 year. The lowest and highest temperatures during the year 1915 

 were 9 C . and 1 6 C . respectively. That Pycnopodia should move 

 up to shallow water during the spawning season seems reasonable, 

 for the temperature in deeper waters is undoubtedly lower than 

 at the surface even during the spring. Reference was made 



