374 TEXTBOOK OF ZOOLOGY |; 



they break through the wall of the gonad into the atrial cavity and 

 pass out through the atriopore with the water. Fertilization occurs 

 in the water. Early summer is the breeding season, and at that time 

 the animals are quite active during the evenings and nights. Fol- 

 lowing fertilization comes a series of cleavage divisions which are 

 total and equal. This is followed by the infolding of one side of the 

 spherical body to form the gastrula and this in turn becomes a free- 

 swimming larva which reaches adult condition without metamor- 

 phosis, only to begin bashfully burying itself in the sand. 



