20 HISTORY OF RESEARCH. 



matic microscope had already placed in their hands a new and powerful instrument of investiga- 

 tion, and discoveries in the anatomy and physiology of the lower animals had begun to accumu- 

 late with a rapidity hitherto unprecedented. We have already referred to the important light which 

 the microscope in the hands of Ehrcnbcrg, Bars, Dalyell, Steenstrup, Dujardin, Leuckart, and 

 Gegenbaur had thrown upon the mutual relations and life-history of these animals, and we have seen 

 the influence it has thus had on their classification, while within the last twelve years the micro- 

 scopical investigation of the Hydroida has been pursued with an assiduity greater than at any 

 former period, and has yielded the results which might have been expected from the numerous 

 able observers who have been engaged in it. To give, however, in the present historical sketch 

 even an imperfect analysis of the various discoveries by which recent anatomical research has 

 enriched our knowledge of the Hydroida, would be impossible without extending our introductory 

 pages far beyond the limits within which it is expedient to confine them, and the reader must 

 accordingly be referred for an account of such discoveries to the parts of this work where they 

 will be specially considered. 



The steps which in the history just sketched have exerted the greatest influence in deter- 

 mining the boundaries and systematic position of the Hydroida, in the sense in which this group 

 is limited in the present Monograph, would seem to be the following : — 



1 . The discoveries of Trembley in the anatomy and physiology of Ihjdra. 



2. The researches of Ellis among the marine forms, which resulted in the complete establish- 

 ment of the animality of the fixed plant-like Hydroida— residts to which the observations of 

 Peysonelle and De Jussieu had led the way. 



3. The establishment of the Radiata by Cuvier as one of the four primary divisions of the 

 animal kingdom. 



4. The recognition by Eschscholtz of two types of form among the Medusae, and his con- 

 sequent subdivision of these organisms into the Phanerocarjm and Cri/ptocarpa. 



5. The recognition by Cuvier, Milne-Edwards, and Rapp, of the difference of structure 

 which separates the actinozoal from the hydrozoal forms of the so-called " Polypi." 



6. The determination by Ehrenberg of the sexuality of the Hydroida. 



7. The discovery of a genetic relation between certain free gymnophthalmic Medusae and 

 the fixed Hydroids, by Loven, Sars, Dalyell, Dujardin, and others, and the correlation of 

 this with analogous phenomena by Steenstrup. 



8. The gradual rectification of the Radiata of Cuvier by the successive elimination of the 

 Nullipores, Worms, Infusoria, Rhizopoda, and Polyzoa. 



9. The further analysis by Leuckart of the amended Radiata, resulting in his establishment 

 of the group Ccelenterata. 



10. The union by Kolliker and Leuckart of certain free gymnophthalmic Medusae with the 

 fixed Hydroida, so as to constitute a single group equivalent in value witli that of the Discophora 

 or steganophthalmic Medusae. 



