442 . CASWELL GRAVE 



tions favorable for development prevail. If the females while 

 spawning did not thus lift their bodies above the sedimentary 

 materials by which they are normally covered, a considerable 

 portion of the eggs might possibly become entangled in the 

 debris and fail to reach the surface. 



The fact that the spawning activities of Ophiura are limited 

 to a definite time of the day, (at or near eight o'clock in the 

 evening) is one for which a satisfactory explanation is wanting. 

 The suggestion at once presents itself that the advent of dark- 

 ness and the changes in temperature which take place soon after 

 sunset, may be regarded as having had something to do in es- 

 tablishing the time for spawning at an early evening hour, but, 

 from our present knowledge of conditions, the advantage to 

 be gained by limiting the activities of spawning to this evening 

 hour does not appear. In this connection it should be borne 

 in mind that Ophiura is a nocturnal species. The coming of 

 darkness is, therefore, a condition which awakens the animals 

 to activities in general, not alone those activities which are 

 connected with reproduction. As soon, therefore, as an Ophiurid, 

 which has its genital bursae filled with mature eggs or sperm, to- 

 zoa, is aroused from the torpor of the day, it is possibly incited 

 to immediate sexual activity by stimuli arising from the ripe 

 sexual elements. Until the mature sexual elements have been 

 extruded, the stimuli to sexual activity arising from them may 

 be considered to overshadow the stimuli to all other activities. 



If, however, this were a complete explanation for the observed 

 fact that Ophiura spawns soon after darkness falls, it should 

 be possible to induce sexually mature animals to spawn at any 

 hour of the day simply by keeping them for a time in a dark 

 cool room, but thus far such an experiment has failed to bring 

 about the expected result. 



