104 



POPULAR ILLUSTRATIONS OF 



by another series of changes, become developed into a vastly-increased 

 number of young Medusae lik'e their original parent ! 



We will follow out these changes by the assistance of Steen- 

 strup's figures in the memoir which first brought the subject prac- 

 tically before the scientific world, although Sars and Chamisso were 

 those by whom the facts of parthenogenesis were originally eluci- 

 dated. Steenstrup's work was translated by Mr. Busk, and pub- 

 lished by the Bay Society in 1845, since which time its illustrations 

 have been generally made use of by naturalists, and very often 

 without the slightest acknowledgment. 



At a period, then, in the existence of the polyp shown at Fig. 137 

 (p. 103), which is unknown, and influenced by the operation of a law 

 which is constant although the nature of the force which brings the 

 law into operation is also unknown this polyp shows a tendency to 

 divide just below the insertion of the tentacles. Instead, however, 



Fig. 138. 



Fig. 139. 



Fig. 140. 



Figp. 141, 142. 



of dividing at once, the body of the polyp increases in size in an 

 upward direction, showing as it does a series of distinct circular 

 markings (Fig. 138). After a time it presents the appearance 

 shown in Fig. 139, where it will be observed each circular marking 

 has become crenated by the growth of processes having an upward 

 direction. The animals or larvae now begin to separate. The first 

 one coming off at a (Fig. 139), which is called by Steenstrup the 

 " polypiform nurse," becomes fixed by a root or sucker-like extension 

 in the centre of the umbrella, and is termed by him a " stationary 

 Medusa." 



Leaving, however, the history of this " stationary Medusa," we 

 find that, after it has separated, the pile assumes the form of 

 Fig. 140, which is nothing more nor less than a series of young 

 Medusae, having now no connection with each other, but which 

 speedily separate and grow up into the full-grown "jelly-fish." 



