148 CAMPANULARIID^E. 



the young zooid, "hardly visible, as very short narrow 

 lines on both sides of part of the upper half of the radia- 

 ting tubes "*. These observations point to variations 

 in the time of development, and prove that the period 

 at which the sporosacs appear is not a point of any special 

 significance. 



In the present work the Eucope and Obelia of Agassiz, 

 embracing respectively the species with 24-armed and 1(3- 

 armed gonozooids, are blended in the single genus Obelia ; 

 and the E. alternata (A. Agassiz) is ranked as a synonym 

 of the well-known and widely distributed 0. geniculata 

 (Linnseus) . 



It appears from the observations of A. Agassiz that, in 

 this genus, " with advancing age the Medusae lose the 

 habit of swimming with the proboscis uppermost, and 

 gradually assume the usual mode of swimming of jelly- 

 fishes." The arms increase greatly and, it would seem, 

 rapidly in number, and the sporosacs become larger and 

 change their form with age, differing somewhat in shape 

 in the two sexes. 



In the Thaumantias of Forbes's Monograph several 

 members of this genus are included; and some of his 

 species are only various stages of one and the same 

 Obelia. 



The earliest figure of the medusiform zooid with 

 which I am acquainted is found in Easter's ' Opuscula 

 Subseciva' (1762)t- He gives a very fair representa- 

 tion of it, and describes it as a polyp which had fallen 

 from the coralline " ob vehementiorem aqua infusionem." 

 He observed a considerable number of these detached 

 polyps swimming freely in the water, and fancied that in 

 some cases they fixed themselves again on the branches 



* North American Acalepha?, p. 78. t Vol. i. p? 27, pi. v. fig. D. 



