PRINCIPLES OF HYDROID ZOOGRAPHY. 237 



In assigning their proper limits to the chissificatory groups under vvliich the gymnoblastic 

 hydroids are here distributed, I have accordingly availed myself of both tropliosome and 

 gonosome in the selection of the characters. As to the gonosome, I have regarded the condition 

 of the gonophores — whether phanerocodonic or adelocodonic, as well as certain important 

 differences in the form of the planoblast, as of primary importance, and have employed these 

 characters in the limitation of families. It is true that there are certain cases which may be 

 viewed as transitional from the adelocodonic to the phanerocodonic gonophore, yet the difference 

 between the medusa and the sporosac is, on the whole, so strongly marked as to afford valual)le 

 characters for the limitation of the higher gronps in any philosophical classification of the 



HVDROIDA. 



It may be objected that to regard the differences between phanerocodonic and adelocodonic 

 gonophores as of so high an order as to justify our employing them in the definitions of families 

 would involve the necessity of widely separating species in all other respects closely resembling 

 one another; that among the calyptoblastic hydroids, for example, the Campanularias, with their 

 phanerocodonic gonophores, and the Laomedeas, with their adelocodonic gonophores, would have 

 to take their places in separate families.' 1 am, nevertheless, prepared to defend such a 

 separation as would be here involved, for I do not, admit the principle that any agreement of 

 the trophosome can justify the depreciation of so very important a difference as is involved in 

 these two conditions of the gonosome. It has been already fully recognised in the comparison 

 of genera, and I believe we are quite justified in employing this well-defined and excellent 

 character as a limiting element in the diagnoses of families. 



While the gonosome will thus afford characters for the diagnoses of the higher groups, 

 characters of corresponding value will also be yielded by the trophosome. These will be derived 

 chiefly from the condition of the tentacles as showing itself in their disposition, whether verticillate 

 or scattered ; and in their form, wliether filiform or terminating in capitula. 



Characters of subordinate importance available for the diagnosis of Genera are also derivable 

 from both gonosome and trophosome. The gonosome will yield them in certain subordinate 

 differences of form in the planoblast, and in the presence or absence of a blastostyle ; while in 

 the trophosome we shall find them in such features as are afforded by the presence or absence of 

 a developed hydrocaulus and by certain minor differences in the disposition of the tentacles of 

 the hydranth. 



Characters which are merely Specific will be found in still more subordinate differences, 

 which are also presented both by gonosome and trophosome, and will easily suggest themselves 

 to the observer. 



With regard to nomenclature, I am convinced that except in cases where a manifestly 

 incorrect determination has been made, and in a few other special cases, we must retain for our 

 hydroid the name under which it was first described, whether this original description refers to 

 the gonosome or to the trophosome. The fact of our assigning to the complete hydroid the 

 generic name by which the planoblast alone had been previously known, needs not prevent our 

 continuing to employ the same name for all those plauoblasts, which do not differ from this in 

 characters of generic importance, but whose trophosomes have not yet been discovered ; we 



^ I refer to the genera Campanularia and Laomedea in the sense in \yhich I have already 

 defined them, and which I see no reason to alter. ' On the Construction and Limitation of Genera 

 among the Hvuhoida,' 'Ann. Nat. Hist.' for May, 1861. 



