INTRODUCTION. 



or altogether wanting. On the return of spring, however, 

 they reappeared. 



The polypites of the Hydroid colony, as well as their 

 raedusiforra brethren, exhibit in many cases the beautiful 

 phenomenon of phosphorescence. This has been observed 

 in many species, but only, I believe, amongst the Theca- 

 phora. Mr. Hassall has celebrated the beauty of the trawl- 

 nets when raised at night, draperied with zoophytes which 

 glitter " like myriads of the brightest diamonds." The 

 luminosity resides in the living polypites, which, when irri- 

 tated, instantaneously light up their little coloured lamps, 

 and literally flash fire at their assailants. The common 

 Obelia geniculata, which may be met with on every coast, is 

 a phosphorescent species, and, if agitated soon after its 

 removal from the sea, will furnish a fine display of " living 

 stars." 



GEOGRAPHICAL DISTRIBUTION. 



Little can be said on this branch of the subject. So far 

 as we can judge, the British Hydroida, with a few excep- 

 tions, are generally distributed round our coasts. There 

 seems to be little localization of species. As yet we have 

 only an imperfect knowledge of the distribution of the 

 smaller kinds ; but the large and well-known species have 

 most of them a very wide range. A few forms are essen- 

 tially boreal and do not descend below the north-eastern 

 section of the English coast : such are Salacia abietina 

 and Sertularella tricuspidata. Sertularia fuse a has only 

 been observed on the east coast of Scotland and the north- 

 east coast of England. 



The eastern sea-board is fully exposed to the sweep of 

 polar currents, which exert a very marked influence on its 



