Chap. II. CLASSIFICATIONS OF ACALEPHS. 151 



HUXLEY'S CLASSIFICATION OF THE SIPHONOPHOE^,, 18.59. 



HYDROZOA. 



I. Calycophorid^. 



l.st Family. Dipliytla; : Diphye?, Galeolaria, Abyla. 



2d Family. Sphoeronectidce : Spha^ronectes. 



3(1 Family. Prayidaj : Praya. 



4th Family. Hippopodiida? : Hippopodiu?, Vogtia. 



II. PlITSOPIIORID^E. 



1st Family. Apolemiad;»? : Apolemia. 



2d Family. StephanomiadEB : Halistemma, Forskalia, Stephanomia, Agalma. 



3d Family. Physoplioriadse : Physopliora. 



4th Family. Athorybiadai : Athorybia. 



5tli Family. Rhizophysiada; : Rhizophysa. 



Gth Family. Physalidas : Physalia. 



7th Family. Velellida; : Velella, Porpita. 



From the circumstance that his last work embraces all the animals then referred 

 to the class, Lesson truly marks the close of a jieriod in the history of the progress 

 of the classification of Acalephs. From his days forward, the improvements bear 

 chiefly upon the arrangement of the Hydroids, first brought into the sphere of 

 attraction of the Medusae about that time. Affinities, unsuspected before, lead to new 

 combinations; and a more intimate acquaintance with the structure of all these ani- 

 mals, by the very novelty of the disclosures, suggests comparisons with the remotest 

 types, and mere analogies are exalted into real affinities. But step by step, the test 

 of homological relationship being applied to these aberrations, and embryological 

 study adding its controlling influence, the Acalephs are finally circmnscribed within 

 limits which would now seem naturtxl, and subdivided into groups which are not 

 likely to undergo other than changes of secondary importance. 



In concluding this rapid sketch of the classifications of the Acalephs I may 

 be permitted to remark, that a retrospective glance over the many attemjDts thus 

 far made to express the various degrees and different kinds of trffinities of these 

 animals, in the shape of diagrams, should satisfy any one how readily difierent 

 authors, approaching the study of these animals with a very difterent preparation, 

 have in the end agreed upon the natural limits of a larger and lai'ger number 

 of their subordinate groups, in proportion as the information concerning these grouj^s 

 has become more and more precise. The disagreement among authors has been 

 most persistent upon the classification of those animals only, respecting the structure 

 of which our knowledge has also remained deficient for a longer period ; and it is 



