72 



the study of the Hydroida or of any of the subdivisions of 

 the order, no better or clearer guide can be recommended. 



The introduction conveys an excellent " general sketch of 

 the structure of the Hydroida, and the history of their repro- 

 duction and development," which, as the author observes, is 

 a fitting prelude to the study of the British species. 



This sketch commences with a terminology, or definition 

 of the terms employed, with description of the various parts 

 or organs, which, in the Hydroida, though neither numerous 

 nor complex, yet, in many respects, are sometimes so obscured 

 by the diff'erent modes in which they are developed, as to 

 cause the uninformed or inattentive observer entirely to 

 overlook their true homologies. By simple adherence, how- 

 ever, to a strict nomenclature, this difficulty is at once almost 

 entirely obviated. As the terms employed by Mr. Hincks 

 are used pretty nearly in the same sense in which they are 

 understood by most naturalists, mainly from the labours of 

 Professor Allman, it is to be hoped that future writers will 

 consent, so far as is possible, to adhere to them, and avoid 

 the temptation which is too often yielded to, of manufactur- 

 ing new ones. 



Mr. Hincks, however, does not always adhere to this list 

 of terms ; as, for instance, in several places when speaking 

 of gonozooids he employs the term spadix for the prolonga- 

 tion of the cajnosarc into the gonozooid, Avhich term is not to 

 be found in his list of definitions. 



The various, and some of them very curious, modes in 

 which reproduction is effected in the Hydroida, have long 

 afforded to naturalists and physiologists a copious field of 

 observation. Although, since the appearance of Steenstrup's 

 work on Alternation of Generations, the main principle, as it 

 may be termed, upon which all these various modifications 

 of the reproductive j^rocess are based, has been more or less 

 distinctly perceived, it is only of late that the entire subject 

 can be said to have been fully elucidated, towards which 

 few, if any, have contributed more largely than Professor 

 Allman. Of this part of the subject Mr. Hincks gives a 

 good condensed account, in which, however, we_ are sur- 

 prised to find that he has said so little on the histo- 

 logy of the tissues, except as regards the thread-cells 

 and nematopliores. Other portions of the introduction 

 contain remarks on the geographical distribution, mode of 

 collecting, including the habitats of different forms of Hy- 

 droida, together with dichotomous tables of arrangement 

 which " are added to enable the student at once to refer any 

 species to its place." Tables of this kind Avould be very 



