CCELENTERATA : HYDROZOA. I I 3 



transitional forms and by an appeal to the phenomena of their 

 development. 



" 3. That many of the so-called Medusidce may, from ana- 

 logy, be regarded as, in like manner, medusiform gonophores. 



" 4. But that there may exist, nevertheless, a group of Medu- 

 sid forms which may give rise by true reproduction to organisms 

 directly resembling their parents, and therefore worthy of 

 being placed in a separate order under the name Medusida" 



The same authority concludes by remarking that to the 

 order as above defined " may be referred provisionally that 

 large assemblage of forms anatomically similar to true Medu- 

 sidcz, but whose development is unknown." Besides the large 

 group of forms thus temporarily admitted, all the Trachynemidce 

 and sEginidce are stated by Gegenbauer to fulfil the conditions 

 of the above definition, and should, therefore, be looked upon 

 as true Medusidce. 



As to the development of these true Medusidce, 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 re- 

 mark 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. 



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

 fact that certain medusiform gonophores are capable of pro- 

 ducing independent forms directly resembling themselves, but 

 this is 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 phenomenon 

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

 Sarsia gemmifera, fig. 31, A), 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, though mostly very diminutive 

 in point of size, are exceedingly elegant and attractive when 

 examined in a living condition, resembling little bells of trans- 

 parent 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 one of the principal sources of the lumi- 

 nosity of the sea. It does not seem, however, that they phos- 

 phoresce, unless irritated or excited in some manner. 



H 



