CCELENTERATA : HYDROZOA. 83 



which surrounds the mouth of the bell. This system of tubes 

 constitutes what is known as the system of the " gonocalycine 

 canals." The gonophore, thus constituted, may remain per- 

 manently attached to the parent organism, as in Tubularia 

 indivisa (fig. 14, c) ; but in other cases still further changes 

 ensue. In the higher forms of development (fig. 14, d) the 



Fig. 14. Reproductive processes of Hydrozoa. a Sporosac ; b Disguised medusoid ; 

 c Attached medusiform gonophore ; d Free medusiform gonophore. The cross shad- 

 ing indicates the reproductive organs, ovaria or spermaria. The part completely 

 black indicates the cavity of the manubrium. and the gonocalycine canals. 



manubrium acquires a mouth at its free, or distal, extremity, 

 and the gonocalyx becomes detached from the parent. The 

 gonophore is now free, and behaves in every respect as an 

 independent being. The gonocalyx is provided with mar- 

 ginal tentacles and with an inward prolongation from its 

 margin, 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 gono- 

 phore is propelled 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 organ- 

 isms which are commonly called "jelly-fishes," and are tech- 

 nically known as Medusa (fig. 15). In fact, as we shall after- 

 wards see, most, if not all, of the gymnophthalmate Medusce, 

 originally described as a distinct order of free-swimming Hy- 

 drozoa, are in truth merely the liberated generative buds, or 

 " medusiform gonophores," of the permanently-rooted Hydroids. 

 Finally, the essential generative elements the ova and sper- 

 matozoa are developed in the walls of the manubrial sac, 

 between its endoderm and ectoderm, and embryos are pro- 

 duced. These embryos, however, instead of resembling the 

 organism which immediately gave them birth, develop them- 

 selves into the fixed Corynid from which the gonophore was 

 produced, thus completing the cycle. 



As we have seen, the generative buds of the Corynida may 



