14 DEVELOPMENT OF 



and one extremity is always thicker than the other. In- 

 ternally they present a cavity, corresponding in form with 

 the external shape. Their movement in the water as 

 well as in the receptacles is effected only by means of 

 cilia, with which their whole external surface is covered 

 (figs. 1 — 5,) and they thus outwardly resemble, rather 

 the ciliated infusoria, such as Leucophrys or Bursaria, 

 than a Medusa. But there is also, in structure and in 

 their manner of moving, a most essential difference be- 

 tween the young fry and the full-grown animal. But 

 since Medusa do proceed from this fry, resembling the 

 free swimming ones, it is evident, that some change must 

 take place either gradually or by a sudden transition 

 {m etamorphosis, transformation . ) 



Let us now trace these young Medusce further. When 

 swimming, the larger extremity is always in advance, 

 which exhibits a depression (fig. 5,) and might readily 

 be supposed to be the anterior extremity, and the de- 

 pression in it to be an oral opening. But it will be 

 immediately observed, that the animal after a short time 

 attaches itself by this end to some fixed object (figs. 7,8,) 

 and the above- described depression then appears as a 

 sort of suctorial disc or organ of attachment which at the 

 same time secretes a viscous, tenacious fluid. The oppo- 

 site extremity then projects from the object to which the 

 animal has attached itself, and a real opening is formed 

 in it, whilst at the same time it becomes larger and more 

 club-shaped (figs. 8, 9.) The opening is at first very 

 small, but soon increases and its surrounding border 

 becomes flexible and moveable and assumes very various 

 forms ; in the contracted state it is quadrangular, and at 

 the four angles may be observed small protuberances, 

 (figs. 10, 11, 12,) which soon increase in length and 

 become slender, and finally, on the fifth or sixth day, 

 shoot out into four long tentacula or arms (figs. 13 — 15.) 

 At a later period, four other tentacula are formed between 

 these, which on the seventh day also appear as small 

 protuberances. Ten days after the little animals have 



