xlvili INTRODUCTION. 



and the Mediterranean. A small group of forms, including 

 a few of the Plumulariidee and some of the most cosmo- 

 politan of the Sertulariida, is common to Britain and 

 South Africa. 



In the South Pacific the Hydroid genera are represented, 

 for the most part, by species distinct from our own ; but a 

 few British forms have been noticed at various points in 

 Australia and the neighbouring seas*. 



METHOD OF COLLECTING THE HYDROIDA. 

 The British Hydroida are all marine, with the exception 

 of Hydra (a truly fluviatile form) and Cordylophom (which 

 is an inhabitant of fresh water here, but elseAvhere is met 

 with in waters more or less saline) . Some of the species 

 are confined to deep water; but a very large proportion 

 are littoral, or inhabit the Laminarian zone, which skirts 

 the shore. On a favourable coast, where a large extent of 

 rock is laid bare at low tide, a rich harvest may be gathered 

 without resorting to the use of the dredge. A large num- 

 ber of the more minute Athecata (Hydr actinia, Corynidce, 

 Atractylida, Tubulariidae, &c.), including many of the 

 rarer and more interesting forms, are littoral in their 

 habits, and also many of the Campanulariidae (the exquisite 

 ' ' Bell Corallines "), of the smaller Plumulariidae, and other 

 Thecaphora. Many species manifest a preference for cer- 

 tain zones of the space included between tide-marks, and 

 are only found within very definite limits. Some are 

 confined to certain kinds of weed, or almost invariably 



* Diphasia pinnata at Sydney (and also in South Africa), PlmntiJaria 

 obliqua in Van Diernen's Land, Sertularia attenuata at Port Adelaide, and 

 Lafoea fruticosa in Bass's Straits. 



Sertularella polyzonias and Scrtularia operculata seem to be the two 

 British species which have the widest range. 



