CCELENTERATA : CORYNIDA. 1 1/ 



which partially closes the mouth of the bell, and is termed the 

 "veil" or "velum." By the contractions of the gonocalyx, 

 which now serves as a natatorial organ, the gonophore is pro- 



Fig. 42. Free-swimming medusiform gonophore of Bougainville 'a superciliaris 

 a fixed Hydroid. Enlarged. (After A. Agassiz. ) 



pelled through the water. The manubrium, with the shape, 

 assumes the functions of a polypite, and its cavity takes upon 

 itself the office of a digestive sac. Growth is rapid, and the 

 gonophore may attain a comparatively gigantic size, being now 

 absolutely identical with one of those organisms which are com- 

 monly called "jelly-fishes," and are technically known as 

 Medusa (fig. 42). In fact, as we shall afterwards see, many 

 of the gymnophthalmate Medusa, originally described as a dis- 

 tinct order of free-swimming Hydrozoa, are in truth merely 

 the liberated generative buds, or " medusiform gonophores," of 

 the permanently rooted Hydroids. Finally, the essential gen- 

 erative elements the ova and spermatozoa are developed in 

 the walls of the manubrial sac, between its endoderm and ecto- 

 derm, and embryos are produced. These embryos, however, 

 instead of resembling the organism which immediately gave 

 them birth, develop themselves into the fixed Corynid from 

 which the gonophore was produced, thus completing the cycle. 

 The swimming-bell of the medusiform gonophore is believed 

 to be formed by a great development of an inter-tentacular 

 web, such as is sometimes present, in a rudimentary form, in 

 the nutritive zooids. Sometimes the medusoid becomes quies- 



