98 MANUAL OF ZOOLOGY. 



" i. That several of the organisms formerly described as 

 MedusidcR are the free gonophores of other orders of Hydrozoa. 



" 2. That the homology of these free gonophores with those 

 simple expansions of the body-wall which in Hydra and some 

 other genera are known to be reproductive organs by their 

 contents alone, is proved alike by the existence of numerous 

 transitional forms and by an appeal to the phenomena of their 

 development. 



" 3. That many of the so-called Medusida may, from analogy, 

 be regarded as, in like manner, medusiform gonophores. 



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

 Medusid 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 Medusida, but 

 whose development is unknown." Besides the large group of 

 forms thus temporarily admitted, all the Trachynemidce and 

 ^Eginidce are stated by Gegenbauer to fulfil the conditions of 

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

 true Medusidcc. 



As to the development of these true Medusida, little is 

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

 ^Eginopsis, 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. 



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

 fact that certain medusiform gonophores are capable of produc- 

 ing independent forms directly resembling themselves, but this 

 is by a process of gemmation, and not by one of true reproduc- 

 tion. Technically these are called " tritozooids," as being 

 derived from organisms which are themselves but the genera- 

 tive zooids of another being. This singular phenomenon has 

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

 gemmifera, fig. 21, 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- 



