68 HTDROZOA. 



in longitudinal folds, which arise, opposite to each 

 other, along the four inner angles of the potypite"s 

 digestive cavity. 



In Pelagia, a permanently free form of the 

 same order, the ova are developed directly into 

 the likeness of the organism within which they 

 are evolved. The young Pelagia, according to 

 Krohn, presents a minute, semi-transparent, some- 

 what cylindrical body, invested in a thin, whitish, 

 ciliated, covering. By means of its cilia, the em- 

 bryo swims rapidly, often turning round on its 

 longitudinal axis. At one extremity, which is 

 truncate, a very small mouth appears, leading 

 into a distinct nutrient cavity, or stomach. This 

 cavity quickly enlarges ; the mouth, also, becomes 

 jDrotruded, whilst at the same time, the hinder 

 end of the body is developed into an umbrella. 

 On the third day, traces of eight lobes indent the 

 margin of the umbrella, an equal number of sacs 

 arising from the sides of the stomach. The mar- 

 ginal lobes lengthen, each becomes further in- 

 dented, and soon rudimentary lithocysts can be 

 distinguished. By this time the oral region is 

 changed into a perceptible polypite, but the or- 

 ganism still moves chiefly by the aid of its cilia, 

 the contractions of the umbrella being at first only 

 occasionally repeated. Afterwards, the ciliated 

 coat disappears ; thread-cells are produced ; the 

 lips of the polypite enlarge ; the umbrella shortens 

 and assumes its proper function ; the crystalline 

 contents of the lithocysts make their appearance. 

 The stomachal sacs increase in number and take 

 on the aspect which they present in the adult 

 animal. Finally, marginal tentacles are acquired, 

 four of these being equal in length to the diameter 



