64 



MARINE ANIMALS OF MASSACHUSETTS BAY. 



Development of Melicertum and Tima. 



In the Melicertum and Tima we have had the good fortune to 

 trace the process by which the eggs are changed into Hydroid 

 communities. If any one has a curiosity to follow for themselves 

 this singular history of alternate generations, the Melicertum is a 

 good subject for the experiment, as it thrives well in confinement. 



Fig. 80. 



After keeping a number of them in a large glass jar for a couple 

 of days at the time of spawning, it will be found that the ovaries, 

 which were at first quite full of eggs, are emptied, and that a num- 

 ber of planulae are swimming about near the bottom of the vessel. 

 After a day or two the outline of these planulae, spherical at first, 

 becomes pear-shaped (see Fig. 81), and presently they attach 

 themselves by the blunt end to the bottom of the jar. (Fig. 82.) 

 Thus their Hydroid life begins ; they elongate gradually, the 

 horny sheath is formed around them, tentacles arise on the upper 



Fig. 80. Melicertum seen in profile ; natural size. 



