THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY, 



[THmD SERIES.] 

 No. 77. MAY 1864. 



XXXV. — On the Construction and Limitation of Genera among 

 the Hydroida. By Prof. Allman, F.R.S. 



It will assuredly seem strange that those principles of classifi- 

 cation which have been acknowledged as the only sound ones, 

 and which have been our guide in the study of every other group 

 of the animal kingdom, should be almost entirely ignored in our 

 attempts at a systematic arrangement of the Hydroida. 



The cause of this, however, is sufficiently obvious. The indi- 

 vidual Hydroid frequently presents itself in disconnected parts, 

 which are very different from one another ; and it is only recently 

 that the researches of zoologists have shown the mutual relation 

 of these parts, and have demonstrated that organisms now en- 

 joying an independent life may have been at one time united in 

 a single individual, and are at all times necessary for an adequate 

 conception of it. So long, however, had the practice prevailed 

 of regarding these component elements of the zoological indivi- 

 dual as if they were entirely independent of one another, that 

 even still we find it more convenient to treat them as such, to 

 assign to them separate places in our systems, and record them 

 under distinct generic and specific names. 



Yet this is totally at variance with the first principles of na- 

 tural classification and of a scientific nomenclatux'e ; and the 

 sooner we get rid of it the better for the harmony of biological 

 method, and the progress of that department of zoology in which 

 it has prevailed. 



For many years it has been known that a considerable num- 

 ber of the fixed Hydroida give origin to buds which detach 

 themselves from the fixed stock, and henceforward lead an in- 

 dependent life in the open sea as free gymnophthalmic Medusse. 

 The first who entertained, or at least gave definite expression to, 

 the true relation between these two sets of elements in the 

 Hydroid seems to have been Dujardin, when he compared the 



Ann. ^Maff.N. Hist, Ser.3. Vol.xiii, 33 



