XXX INTRODUCTION. 



iii autumn certain species swarm in immense shoals off 

 the coast *. Any one who has watched the escape of the 

 goiiozooids from a specimen of the common Obelia genicu- 

 luta will feel no surprise at the accounts which are given 

 of the numbers of minute medusas that sport near the 

 surface of the ocean, and at night make it glow with 

 phosphoric fires. 



In this species each of the pretty urn-like capsules, 

 which are produced in great profusion, contains a large 

 number of zooids ; and several hundreds are soon liberated 

 even from a small specimen. On the vast tangle-beds that 

 fringe all portions of our coast, Obelia geniculata is uni- 

 versally present, forming extensive forests over the long 

 ribbon-like fronds ; and from these, during the breeding- 

 season, countless thousands must be cast off. 



The stationary life of the polypi te does not offer much 

 to interest the observer ; but the habits of the medusiform 

 zooids are singularly attractive. 



Like miniature balloons they float suspended in the 

 water for awhile ; then they suddenly start into motion, 

 propelling themselves by a series of vigorous jerks or casts, 

 and at the same time contracting the tentacles into the 

 smallest compass ; then they become quiescent again, and 

 sink slowly and gracefully, like parachutes, to the bottom 

 of the vessel, some of the arms extended laterally, and the 

 rest dependent. In all cases locomotion is effected by the 

 pulsation (the alternate systole and diastole) of the swim- 

 ming-bell. 



The tentacles have various uses. They assist in the 

 capture of prey ; they are employed as organs of attach- 



* A. Ayassiz. ' Catalogue of North American Acalepba 1 ,' p. 7-'!. 



