130 .. MANUAL OF ZOOLOGY. ) 



> From the above description it will be evident that the 

 Medusa is in all essential respects identical in structure with 

 the free- swimming generative bud or gonophore of many of 

 the fixed and oceanic Hydrozoa. Indeed, a great many forms 

 which were previously included in the Medusida, 'have now 

 been proved to be really of this nature ; and it is only in a 

 comparatively small number of Medusa that the direct mode of 

 development which alone would entitle them to be ranked as 

 independent organisms has been observed. .The Trachynemida, 

 sEginidce, and Geryonidcz appear, however, to be directly de- 

 veloped from the ovum, and are therefore properly placed in 

 the present order; while we may temporarily include here a 

 number of Medusoid forms, the development of which is at 

 present unknown. 



As to the development of these true Medusidcz, little is 

 known for certain. It appears, however, that in Trachynema, 

 sEginopsis, and other genera, the embryo is directly developed 

 into a form resembling its parent, without passing through any 

 intermediate changes of form. It is hardly necessary to remark 

 that this is not the case with the embryos of a medusiform 

 gonophore, these being developed into the sexless Hydrozoon 

 by which the medusoid was produced. It has also been shown 

 that various of the true Medusa (Cunina, dZgineta, &c.), have 

 the power of producing new forms like themselves by a process 

 of budding, the phenomena attendant upon this being some- 

 times of great interest. 



In this connection, allusion may be made to the long-known 

 fact that certain " medusiform gonophores " are likewise 

 capable of producing independent forms directly resembling 

 themselves by a process of gemmation, and not by one of true 

 reproduction. Technically these are called " tritozooids," as 

 being derived from organisms which are themselves but the 

 generative zooids of another being. This singular phenome- 

 non has been observed in various medusiform gonophores (e.g., 

 Sarsia gemmifera), the buds springing in different species from 

 the gonocalycine canals, from the tentacles, or from the sides 

 of the polypite or manubrium. 



The "naked-eyed" Medusa and their allies the "medusi- 

 form gonophores," though mostly very diminutive in point of 

 size, are exceedingly elegant and attractive when examined in 

 a living condition, resembling little bells of transparent glass 

 adorned here and there with the most brilliant colours. They 

 occur in their proper localities and at proper seasons in the 

 most enormous numbers. They are mostly phosphorescent, 

 or capable of giving out light at night, and they appear to be 



