THE GONOSOME. 29 



cells and arc swnouiidecl at their base by a cliitinous sheath. During tlie life of tlie liydroid tlicy 

 may be seen to be in a state of great activity, stretching themselves out and twisting about in 

 every direction. 



III. MoRVnOI.OGT OF THE GONOSOME. 



1. General view of the Gonosome. 



The zooids which compose the gonosome may remain permanently attached to the rest of the 

 hydrosome, or they may become free and lead henceforth an indc'ijcndent existence in the open 

 sea. It will be found very convenient to have a common term to express all these free zooids of 

 the gonosome, and I accordingly propose for them the name of planohlast} With one rare 

 exception, they are all in the form of the so-called gymnophthalniic medusas. 



Under the planoblasts, however, must be included certain comparatively rare instances in 

 which the medusiform zooid, though having its natatory organ well developed, remains, from 

 some unknown cause, attached to the trophosome, and attains to sexual maturity without ever 

 actually becoming free. It is capable, however, when accidentally detached, of swimming by the 

 systole and diastole of a true natatory umbrella, and cannot, therefore, be placed in a different 

 category from that of the essentially free planoblast. 



The gonophore is the uHimcde (generative zooid, — that on which devolves the duty of giving 

 immediate origin to the generative elements which are always produced from it between its own 

 ectoderm and endoderm without the intervention of any other zooid. It is the only essential 

 part of the gonosome, being never absent. It may remain attached during its whole lifetime, or 

 it may sooner or later separate itself from the rest of the hydrosome, and thus become a free 

 generative bud or planoblast. It presents either the form of a " gymnophthalmic medusa" or 

 else that of a zooid in which the form of the medusa is more or less disguised, or its parts 

 more or less suppressed, but which can nevertheless be always referred by an easy comparison to 

 the essential type of the medusa. 



The planoblasts may either be the direct producers of the generative elements, and are then 

 true gonophores, or may never produce the generative elements directly, but only through the 

 intervention of another bud which is developed from them. For this latter form of planoblast, 

 which is thus, strictly speaking, non-sexual, and which, notwithstanding its resemblance to a 

 gonophore, should be carefully distinguished from it, I have given the name of blastocheme ; while 

 the proper sexual medusiform planoblast may be designated by the term ^onoc/ieme. 



Among the fixed zooids of the gonosome there is very frequently one which, like the blasto- 

 cheme, takes only a subordinate part in the generative function, being, like it, destined for the pro- 

 duction of other generative buds which are developed from some part of its sm-face, and which 

 may be either gonophores or blastochemes. It is never medusiform, but may be regarded as a 

 peculiarly modified hydranth, having its alimentary function suppressed. It is referred to in the 

 present work under the name of hlastostyle. 



The gonophore is always borne as a bud, either directly upon some part of the trophosome, 

 or upon a blastostyle, or upon a blastocheme. While it is constructed essentially on the plan of 

 a gymnophthalmic medusa, it varies greatly in the degree of completeness in which this plan is 

 expressed in it. It may be referred to one or other of two principal types, based respectively on 



' " Waudering buds " — TrXavo^ai and /BAaaroc. 



