CCELENTERATA : HYDKOZOA. 87 



assemblage of forms anatomically similar to true Medusidce, 

 but whose development is unknown.' Besides the large group 

 of forms thus temporarily admitted, all the Trachynemidce and 

 JEyinidcB 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, 

 JEginopsis 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 medusi- 

 form gonophore, these being developed into the sexless Hy- 

 drozoon 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 re- 

 production. 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), the buds springing in different species 

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

 sides of the polypite or manubrium. 



CHAPTER XI. 



LUCEBNABIDA AND GEAPTOLITIDM. 



SUB-CLASS IV. LUCERNAEIDA (Acalephce, in part). The mem- 

 bers of this sub-class may be defined as Hydrozoa, ' whose hydro- 

 soma has its base developed into an "umbrella," in the walls 

 of which the reproductive organs are produced.' (Greene.) 



A large number of forms included in the Lucernarida were 

 described by Edward Forbes under the name of Steganophthal- 

 mate Medusce, being in many external characters closely simi- 

 lar to the Medusidce. This resemblance is especially strong 

 between the disc or ' nectocalyx ' of the true Medusidce and 

 the ' umbrella ' of the Lucernarida, the latter being often a 

 bell-shaped swimming organ, with marginal tentacles, and 

 containing one or more polypites. These analogous structures 

 (Jigs. 20 and 28) are, however, distinguished as follows : 



