372 TEANSACTIONS LIVEEPOOL BIOLOGICAL SOCIETY. 



This report can only be regarded as a preliminary one, 

 as it is simply an abstract of some of my notes. I hope 

 at a later date to publish an accomit of each species with 

 figures. The great obstacle which prevents progress is 

 not being able to collect sufficient specimens of a species 

 so as to trace its development and to see how far it is 

 necessary to allow for variation. So far my attempts to 

 rear medusae from their hydroids, or to keep them in 

 confinement have been failures. The rearing of a medusa 

 in confinement is the only safe method for tracing its 

 life-history, and until this can be done the uncertain 

 method of catching a sufficient quantity of specimens 

 must be adopted. Some species have a characteristic 

 feature, like the eight radial canals of Melicertidium, 

 which distinguish them from other allied species, and by 

 which it is easy to identify them at any stage in develop- 

 ment. But unfortunately the majority of species have 

 not such characteristic features, and it is difficult to 

 distinguish the early stages of different species. The old 

 genus Thaumantias of Forbes, which has been divided 

 into many families and genera by Haeckel, still remains a 

 great stumbling-block to the medusa-collector. Another 

 difficulty which has yet to be overcome is the final indenti- 

 fication of a number of doubtful species of hydroids, which 

 are known to bud off medusae. When the life-histories 

 of the hydroids with their medusae are known and the 

 specific differences correctly determined there will be no 

 difficulty in adopting a single system of classification. 

 At present two systems exist, one for the hydroids and 

 another for the medusae. I have used in this report 

 Haeckel' s system of classification and the names of his 

 genera, but have also given the principal synonyms and 

 references to those species which are mentioned by Forbes, 

 Hincks and Allman. There is no need in this kind of 



