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ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 359 
Though it is necessary to distinguish the gonoblastidia from the polypites, 
it cannot be overlooked that they pass into them by certain transitions. 
Agassiz* describes a well-developed mouth in the so-called fertile polypites 
or gonoblastidia of the Hydractinia polyclina of the North American coast, 
and it is doubtful whether even in our Hydractinia echinata the nutritive 
function is ever absolutely suppressed in the otherwise very characteristic 
gonoblastidia of this species. In certain Hudendria the polypites, which 
carry the gonophores grouped round their base, present a perfectly developed 
form while the gonophores are young; but as these continue to grow, the 
polypites which carry them frequently become atrophied, losing their ten- 
tacles and mouth, and by the time the gonophores have attained maturity 
the polypites have assumed the condition of gonoblastidia. Again, among 
the Sertularida, we find in Haleciwm halecinwm the gonoblastidium, which 
here becomes a “ blastostyle’’ (see p. 370), developing from its summit a pair 
of perfect polypites with tentacles and mouth. These, however, are all ex- 
ceptional cases, and do not render less valid the association of the gonoblas- 
tidium with the gonosome rather than with the trophosome, while they are 
important as showing the homological identity between the polypite and the 
gonoblastidium. 
When we compare with one another the various forms of phanerocodonic 
gonophores, we shall see that they are divisible into two important groups. 
In the gonophores belonging to one of these groups the generative elements 
are produced directly by the gonophore itself, being developed between the 
endoderm and ectoderm of its manubrium (fig. 8, p. 369); while in those be- 
longing to the other group they are not produced directly by the gonophore, 
which is then, properly speaking, non-sexual, but are found in a special sexual 
bud to which this non-sexual gonophore itself gives rise (figs. 17 & 18, p. 401). 
It is necessary to distinguish these groups; and I shall accordingly designate 
the former or sexual form of phanerocodonic gonophore by the name of gono- 
cheme+, while to the latter I shall give the name of gonoblastochemet. 
Whether the entire gonosome remains during its whole lifetime connected 
with the trophosome, or becomes in any part an independent zooid,. it is 
manifest that it constitutes an essential element in the character of the spe- 
cies, and the study of no one species of hydroid can be regarded as complete 
unless it embrace both trophosome and gonosome. Since, however, in many 
cases we are acquainted with only the free gonophore, not having yet dis- 
covered the trophosome to which it belongs, while in other cases the tropho- 
some alone is known to us, we have been in the habit of treating such in- 
stances in our systems without regard to the missing zooids, and as if they 
afforded examples of independent species; but it must never be forgotten 
that the data on which we thus assign to them the rank of determinate 
_ species, or even genera, are insufficient for the purposes of a philosophic 
system: such genera and species must be regarded as purely provisional ; 
for the zoologist is no more justified in accepting such incomplete characters 
as sufficient for the accurate determination of his hydroid, than would the 
botanist be in regarding the flower alone on the one hand, or the root, stem, 
_ and leaves alone, on the other, as affording characters sufficient for the defi- 
_ nition of any flowering plant whose exact determination he would attempt. 
* Contrib. to the Nat. Hist. of the United States, vol. iv. 
+ Yovos, offspring, and éynpa, a carriage. 
{t Tovos, BXaorés (a bud), and 6ynpa. 
