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Johnston ; but, in addition to these, the term as now under- 

 stood, necessarily includes most, if not all, of the so-called 

 naked-eyed or gymnophthalmic medusae, Avhose relation to 

 the fixed forms was unknown to the distinguished author 

 of ' British Zoophytes.' The term, therefore, is understood 

 in the same sense as it is by Mr. Hincks in his excellent 

 ' History of British Hydroid Zoophytes.' 



Professor Allman commences his work with a very full 

 account of the history of the progress of our knowledge of 

 the Hydroida, but which, for reasons given, is in great 

 measure confined to the more important steps which have 

 been made towards the determination of their systematic 

 position, and their recognition as a distinct group. In 

 this exhaustive chapter much interesting matter will be 

 found, not only in an historical point of view, but as con- 

 veying, in the account of the conflicting notions of various 

 observers, very important information respecting the mor- 

 phology and physiology of the Hydrozoa in general. 



The next chapter is devoted to the morphology of the 

 hydroids and the explanation of the terminology adopted by 

 Professor Allman, which will, it is to be hoped, receive 

 universal acceptation ; for, when thoroughly comprehended, 

 the terms employed will serve at once to convey a clear 

 and precise notion of the nature and morphological signifi- 

 cance of the multitudinous elements which enter, at one stage 

 or another, into the complicated life history of a hydroid. 



In the main, the terminology corresponds with that em- 

 ployed by Mr. Hincks, but with many additions, serving to 

 render descriptions more precise and intelligible. 



The sense, moreover, in which some of the terms are 

 employed differs slightly from that usually attached to them. 

 As, for instance. Dr. Allman assigns the following characters 

 as distinguishing a " zooid" from an " organ" (a distinction, 

 as he observes, which it is not in all cases very easy to make). 

 "A zooid," he says, "is a more or less individualised animal 

 organism, which may or may not be capable of independent 

 existence, and which constitutes one of a series whose mem- 

 bers are related to each other by some form of non-sexual 

 reproduction, and morphologically repeat one another either 

 actually or homologically." 



" In this sense," he continues, " not only are the free 

 medusiform buds of the hydroida true zooids, but we must 

 also regard as such the fixed hydranths and those fixed 

 gonophores which never attain a developed medusiform 

 structure, as well as the single generative sacs which are 

 developed in the radiating canals of Obelia, Thaumantias, &c. 



