luith Observations on Australian Hydroids. 75 



its proper sub-family by its general facies, and by the pre- 

 dominance of the characters of one group over those of the 

 other. 



The prevalence in the old genus Aglaophenia of special 

 ramuli, armed with nematophores, and serving to protect 

 the gonangia, has long been familiar, and of late years two 

 genera of Hydroids have been made known which combine 

 such gonangial structures with the trophosome of a typical 

 Plumularia. Professor Allman accordingly divides each of 

 the sub-families Eleutheroplea and Statoplea into two 

 minor groups — the Phylactocarpa, which have the gonangia 

 protected by some form of " phylactocarp," or special ramulus 

 armed with nematophores ; and the Gymnocarpa, which are 

 destitute of any such structures. The phylactocarpal 

 ramuli of Aglaophenia and its allies are the " nematocladia" 

 of Kirchenpauer. 



Note on the Inteathecal Ridge. 



With regard to the structure of the hydrotheca in the 

 Plumulariidge, the " Challenger" Report says : — " In almost 

 every case there is present in the hydrotheca of the Stato- 

 plea a slightly projecting chitinous ridge, which runs on the 

 inner surface of the walls transversely from behind forwards, 

 but with a more or less oblique direction, and which more 

 or less completely encircles the cavity of the hydrotheca, 

 thus forming an imperfect septum, which divides the h^^dro- 

 theca into a proximal and a distal portion. This is the 

 intrathecal ridge^ which, as just said, is never present in the 

 Eleutheroplea. In some species of Statoplea, what has the 

 appearance of a similar ridge running from before backwards 

 may be seen in the anterior portion of the hydrotheca. 

 This, however, is only the optical expression of a fold in the 

 walls of the hydrotheca." So far as the Australian Plumu- 

 lariidse are concerned, however, the condition is just the 

 opposite of that stated in the foregoing extract, and the true 

 intrathecal ridge, or partial septum, is that which runs from 

 before backwards, while the posterior one is generally a 

 mere fold in the walls of the hydrotheca. In the half-dozen 

 species of which Halicornaria siiperha is the t^^pe,* and 

 also in Aglaophenia Huxley i and A. phoenicea (both of 



* In H. speciosa, a species of this group figured in the Eeport on the Gulf 

 Stream Hydroids, the anterior ridge is described by Professor Allman as the 

 * ' intrathecal ridge. " • 



