FORMATION OF " STROBILA." 83 



In the course of a short time the segments thus 

 formed become surrounded with marginal rays dicho- 

 tomously divided at their extremities. These arms or 

 rays are free, all having their apices turned in the 

 same direction,, and disposed with such regularity, 

 that the once polyp-like body seems to be furnished 

 with eight longitudinal ribs (PL I. fig. 5, e), and now 

 evidently consists of a series of discs placed one upon 

 the other, like a pile of soup-plates or saucers ; but as 

 those nearest the top of the pile grow more rapidly 

 than the rest, the creature ultimately assumes a 

 conical shape, somewhat resembling a champagne- 

 glass (PL I. fig. 5,/), or, as a lady observes of some 

 specimens now before us, " they look exactly like the 

 tassels of our dimity curtains/' In this condition 

 the Hydra tuba is the far-famed Strobila, whose re- 

 markable history, first discovered and described by 

 M. Sars in the year 1829, formed an epoch in zoo- 

 logical science. 



And here comes the most wonderful part of the 

 proceeding : the uppermost disc of the Strobila, now 

 nearly mature, begins to exhibit very distinctly the 

 form of a Medusa, and after sundry tugs and pulls, as 

 though impatient of restraint, breaks loose from the 

 top of the pile (PL I. fig. 5, h), and escaping into the 

 surrounding water, swims away, presenting all the 

 characters and attributes of a free Acaleph, and in this 

 condition assumes an independent existence under the 

 appearance represented in the figure (PL I. fig. 5,^). 



A group of twelve individuals, just on the point 

 of separation, measured about one-third of an inch 

 in length, and when examined by the microscope, 



