74 MANUAL OF ZOOLOGY. 



tubes, which run to the margin of the bell, where they com- 

 municate with one another by means of a single circular canal 

 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 

 indimsa (Jig. 14 c) ; but in other cases still further changes 

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

 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 

 organisms which are commonly called 'jelly-fishes,' and are 

 technically known as Medusce (fig. 20 a). In fact, as we 

 shall afterwards see, most, if not all, of the gymnophthalmate 

 Medusce, originally described as a distinct order of free- 

 swimming Hydrozoa, are in truth merely the liberated genera- 

 tive buds, or ' medusiform gonophores,' of the permanently- 

 rooted Hydroids. Finally, the essential generative elements 

 the ova and spermatozoa are developed in the walls of the 

 manubrial sac, between its endoderm and ectoderm, and 

 embryos are produced. These embryos, however, instead of 

 resembling the organism which immediately gave them birth, 

 develop themselves into the fixed Corymd from which the 

 gonophore was produced, thus completing the cycle. 



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

 exist in the following forms: 1. As 'sporosacs,' or simple 

 closed sacs, consisting of ectoderm and endoderm, with a 

 central cavity in which ova and spermatozoa are produced. 

 2. As ' disguised medusoids,' in which there is a central 

 manubrial process and a rudimentary system of gonocalycine 

 canals ; but the gonocalyx remains closed. 3. As complete 

 medusoids, which have a central manubrium, a complete 

 system of gonocalycine canals, and an open gonocalyx ; but 

 which never become detached. 4. As perfect medusiform 

 gonophores, which are detached, and lead an independent 

 existence for a time, until the generative elements are matured. 



