CGELENTERATA : HYDROZOA. 



95 



like the clapper of a bell, there depends a peduncle or 

 " manubrium," which contains a process of the somatic cavity. 

 The manubrium gives out at its fixed or proximal end four 

 prolongations of its cavity, in the form of radiating lateral 

 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 

 constitute 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. 20, c] ; but in other cases still further changes 

 ensue. In the higher forms of development (fig. 20, d) the 



Fig. 20. 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 marginal 

 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 gonophore is pro- 

 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. 21). In fact, as we shall afterwards see, most, if not all of 

 the gymnophthalmate Medusa, originally described as a distinct 

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

 liberated generative 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 



