Ixiv INTRODUCTION. 



genera Syncoryne and Periffonimus, and more or less so in 

 several others. In such groups the alimentary portions of 

 structure would seem to have been more susceptible of 

 modification than the reproductive. 



In constituting the genera, I have endeavoured to give 

 due weight to the different structural elements. I have 

 followed Agassiz and Allman in regarding the presence or 

 absence of a free sexual zooid as a character of generic 

 value, though the adoption of this view leads to the sepa- 

 ration of species that in all else are most nearly allied. 

 But I must most strongly dissent from the practice of 

 those authors who have multiplied divisions on the ground 

 of slight variations in the gonozooid*. 



The three suborders under which I have distributed the 

 British Hydroida correspond with the Tubularina, the 

 Sertularina, and the Hydrina of Johnston ; but I have- 

 thought it better to introduce significant titles for these 

 higher divisions rather than to ring the changes on the 

 names of the typical genera. 



The character which distinguishes the first suborder, 

 Athecata (the naked condition of the polypites), is asso- 

 ciated with great diversity in the configuration of the body 

 and the structure and disposition of the tentacles. A rich 

 variety of shape and colour characterizes the polypites of 



* There has been a tendency amongst some writers to pay almost exclu- 

 sive attention -to the medusan element, both in their description and classifi- 

 cation ; but the nutritive and reproductive structures are coordinate, and 

 due regard must be had to both, if we are to form a just conception of the 

 individual Hydroid, or of the affinities and relationships of the Hydroida. 



On the subject of classification, reference may be made to two admirable 

 and exhaustive papers in the ' Natural History Review,' Nos. xi. and xii., for 

 July and October, 1863, which are devoted to a review of the 4th vol. of 

 Agassiz's ' Contributions,' and discuss very fully and with great ability the 

 various questions connected with the systematic arrangement of the Hydrozoa. 



