THE GENERA AND SPECIES OE THE 



GYMNOBLASTEA. 



Genkkal Principles of Hydrotd Zoogrmmiy. 



WniLE the first part of this Monograph is devoted to those general questions of hydroid 

 organization and Hfe whieh concern the IIydroida as a whole, I purpose in this second part to 

 deal with the zoography or descriptive and systematic zoology of the various genera and species 

 which are eud)raced under the Gymnoblastea, that great section of the Hydroida to which the 

 special part of the iMonograph must be confined.* 



It is but quite lately that those princijjles of classification which are acknowledged as the 

 only sound ones, and which have been our guide in the study of every other group of the 

 animal kingdom, had begun to be recognised in the systematic treatment of the Hydroida. 



The cause of this long exclusion of the Hydroida from tiie douiain of a philosophic 

 classification is sufficiently obvious. The individual hydroid, as we now know, frequently 

 presents itself in disconnected parts which are veiy different from one another, and it is only 

 recently that zoologists have shown the mutual relations of these parts, and have demonstrated 

 that organisms totally different from one another in form, and now enjoying an independent life, 

 may have been at one time united into a single individual of which they formed essential 

 constituents, and are at all times necessary for an adequate conception of it. 



So long, however, had these component elements of the zoological individual been regarded 

 as entirely independent of one another, that even after their true relations became known 

 zoologists continued to find it more convenient to treat them as independent organisms, to 

 assign to them separate places in their systems, and to record them under distinct generic and 

 specific names. 



A practice, however, so totally at variance with the first principles of a philosophic 

 classification and of a scientific nomenclature could not last. With the possible exception of 

 the MoNOPSEA, in which no hydriform trophosorae exists," the individual hydroid can only be 



' See Part I, pages 188 — 191, for the systematic position of the Hydroida, and the leading 

 diagnoses of the larger groups. 



^ I say, " with the possible exception," for even in the Monopsea we ought probably, as already 

 pointed out (see p. lOG), to recognise the existence of the two zooidal elements, tro[)hosome and 

 gonosome ; here, however, the medusiform condition of the trophosome and its permanent uuiou with 

 the gonosome will be sure to secure to both a full consideration iu any diagnosis. 



Victor Carus, iu his excellent treatise on the Hvdkoida (Carus und Gerstaecker, ' Handbuch der 

 Zoologie,' vol. ii), has proposed the name of Haplomorpha for the group to which I have in the 

 former part of this Monograpli applied that of Monopsea. There is no possible objection to the name of 

 Haplomorpha; and were it not that I had accidentally overlooked its existence, I should not liave 

 ventured to substitute another for it. 



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