The Development of Phascolosoma. 35 



sometimes nearly to the posterior extremity of the body cavity. 

 Into this fluid the reproductive cells are then swept. 



Spermatozoa and ova are forcibly ejected into the surrounding 



sea water through the nephridiopores in a cloud-like jet. Eg-g-laying 



is preceded by the shooting forth of sperm by the males in the 



vicinity. Always active at night, during the ejection of clouds of 



sperm they make vigorous movements, raising the anterior part of 



the body from the bottom of the aquarium and swinging it back 



and forth, so that the jets from the nephridiopores meet with no 



obstruction. The touch of the spermatic fluid diffused through the 



water excites the females to eject the contents of their nephridia. 



The usual period of egg laying begins about 8—9 p. m., and 



extends to 4—5 a. m. By keeping specimens during the daytime 



in an aquarium covered with a dark box, which excluded the light 



very effectuall}^, I observed that in two cases eggs were laid between 



6—7 p. m. and once about 3 p. m. On two occassions egg-laying 



was apparently inhibited by taking the specimens from the dark 



chamber and exposing them to the twilight, since they laid no eggs, 



while others of the same lot, which had not been covered, laid 



abundantly. In three cases which were favorable for observation, 



the nephridia of the specimens kept in the dark during the day 



were more distended than those of others which had been exposed 



to the light. 



From these observations we may conclude that general relaxation 

 of the body, caused by darkness, brings about a distention of the 

 nephridia with fluid, an action which is preliminary to the filling 

 with eggs or spermatozoa; that the time of egg-laying can be 

 hastened only very slightly by keeping the animals in the dark; 

 that removal from extreme darkness to twilight may cause a 

 suspension or inhibition of the process of egg-laying; and finally 

 that there is an established rhythm in this action, which is to a 

 certain extent independent at present of external surroundings. In 

 this connection may be mentioned the changes in color which the 

 prawn Hippolyte undergoes twice daily, being azure blue at night 

 and motley-lmed during the day. These color-changes have been 

 found by Gamble & Keeble (1900) to be so established in periodicity 

 that they occur when the specimens are kept continuously in light 

 or in darkness. 



