626 PHOTOPERIODISM IN INVERTEBRATES 



environmental factor or factors the cycle is correlated, and (4) 

 ultimately to try to regulate the cycle by experimental variation of the 

 factor or factors concerned. To date, only the first two objectives have 

 been accomplished for a small number of species, and the third has 

 been attempted. Experiments to vary the factors of the environment 

 are being planned for several species, and one series is under way 

 using the ochre star, Pisaster ochraceus. 



The main difficulty in achieving the fourth objective is the inability 

 to keep many of the marine animals ahve for prolonged periods 

 of time, even in aquaria with running sea water. Conditions are not 

 natural, and the animals visibly sicken after several months and many 

 die. An alternative is to attempt to modify conditions in the natural 

 environment, but the magnitude of the ocean and the violence of 

 winter storms makes tinkering even with inlets of the sea inadvisable. 

 However, since the same species may occupy a wide range on the 

 coast, one factor, namely temperature, is varied in nature and it may 

 serve to give information on the relation between temperature and 

 the gonadal cycle if specimens are collected from widely divergent 

 habitats. At the present time the purple sea urchin, Strongylocentrotus 

 purpuratus, is being sampled at Los Angeles, at Yankee Point near 

 Monterey, and at Moss Beach near San Francisco. 



The survey has had to be confined to relatively sessile or homing 

 animals since an attempt to determine the gonadal cycle of the squid, 

 Loligo opalescens, has emphasized to us the difficulty of studying such 

 an active form. The gonads always seemed to be well developed in 

 the specimens caught in Monterey Bay. It now appears that the 

 animals come into the bay only to spawn. 



MATERIALS AND METHODS 



For the present, investigation has been confined to representatives 

 of three phyla — the echinoderms, the mollusks, and the arthropods 

 (crustaceans). A study of some of the worms is planned, and ulti- 

 mately representatives of other phyla will be examined as well. 



The species of echinoderms which have been investigated are 

 Pisaster ochraceous (the ochre star), Pisaster giganteiis (the giant 

 starfish), Pateria miniata (the sea bat), Strongylocentrotus purpuratus 



