allman's gymnoblastic hydroids. 65 



This is, no doubt, the true view of the case, though the last 

 instance given may appear, at first sight, a little staggering. 



The morphology of the hydroida is considered under these 

 two heads : 1st, of that of the tropJiosome, or collection of 

 nutritive zooids, or the hydrmiths, and the stem upon which 

 thy are seated — the hydrophijton ; and, 2nd, of the gonosome, 

 or aggregate of reproductive zooids (gonophores), &c. These 

 zooids may remain permanently attached to the rest of the 

 hydrosome, or may become free; for which latter class, 

 represented by the naked-eyed medusae, the author proposes 

 the general appellation of jjlanoblasts, or " wandering buds." 

 Some of these, however, although presenting all the essential 

 characters of a free medusiform zooid, may remain attached 

 to the parent zooid during the whole of their existence. 



But these planoplasts are nevertheless themselves distin- 

 guishable into two distinct kinds, in one of which the 

 generative elements are directly produced — true gonopliores — 

 or indirectly, " through the intervention of another bud 

 which is developed from them." For the latter form of what 

 may be termed non-sexual gonophores, the designation of 

 hlastocheme is employed, Avliilst the proper sexual medusi- 

 form planohlast is designated a gonocheme. 



As " the gonosome is that part of the hydroid which pre- 

 sents the most marked variation among the different membeis 

 of the group," the study of the various forms it presents 

 '* possesses for tha morphological student a profound signi- 

 ficance." 



The author, consequently, enters upon a detailed examina- 

 tion of the various modifications of the gonosome, in the 

 most exhaustive manner, whilst his descriptions of the 

 complicated structures concerned are illustrated with an 

 abundance of excellent woodcuts, the whole constituting the 

 most complete and most intelligible account of this difficult 

 subject anywhere to be found, and bringing our knowledge 

 of it, in every particular, up to the latest period. 



The same may be said of the next chapter — on develop- 

 ment ; but space will not allow us here to give even a 

 superficial account of these most interesting portions of 

 Dr. Allman's labours, and we shall merely cite the two 

 fundamental propositions to which the study of the develop- 

 ment of the hydroids has led : 



" 1. The fixed plant-like hydroida give origin to sexual 

 buds not only in the form of closed sacs, which develope 

 within them the generative elements ; but also in that of a 

 more specialised form of bud which becomes a free (rarely 

 fixed) medusa, which ultimately attains, either directly 



VOL. XII. NEW SER. E 



