230 BULLETIN OF THE 
whole head modified into the form of an elongated axis or stem, By these 
changes the clusters of grape-like organs would appear as lateral branches of a 
main stem ; and if we suppose the clusters of gonophores pushed out to their 
tips, we should have an exact resemblance to the condition of the gonosomes * 
of Hydrichthys, where they are simply botryoidal clusters of immature medusee 
mounted on peduncles which arise from a common stalk. How is it with the 
filiform bodies of Hydrichthys? In reply, it may be said these do not occur 
in Tubularia, Morphologically, they may be supposed to be the single simple 
hydroid, stripped of tentacles, gonophores, and enveloping sheath, so that the 
axis alone, with its terminal opening, is about all that remains. By this 
reduction we have one of the simplest forms of hydroids. Such an individual 
is certainly as low in organization as the Protohydra, Microhydra, and similar 
low genera which are destitute of tentacles. 
This reduction in the form of the hydranth by the disappearance of the ten- 
tacles in Hydrichthys is believed to be a degeneration brought about by its 
life, and not, as in Protohydra, due to the low zoélogical position of the hy- 
droid.t The character of the medusa of Hydrichthys and its resemblance to 
* As this comparison is only in general external outlines, no account of the 
fact that the gonophores of Tubularia take the form of actinulew, while those of 
Hydrichthys appear as medusz, is considered. Hn passant, however, it might be 
said that morphologically the actinula and the medusa are thought to be homolo- 
gous, as several naturalists have already shown. I regard both medusa and hy- 
droid as a modification in different directions of an ancestral form which is most 
closely adhered to in a stage of the Siphonophores to which I have given the name 
“primitive larva,” or “ primitive medusa.” Morphologically considered, a medusa 
and a simple hydroid are homologous, as shown by a study of Stephanoscyphus ~ 
(Allman), Cunina, the young of Agalma as compared with the young Nanomia, and 
other genera. This identity, in a morphological way, of medusa and hydroid has 
long been recognized, and was pointed out many years ago by Claus and others. 
The egg in its development may pass into one or the other of these homologous 
stages. It may become fixed to a submarine object, and become a fixed hydroid; 
it may pass into a free medusa or medusiform condition homologous to a hydroid, 
as in Glossocodon or in Agalma; or it may be developed into a parasitic Hydrich- 
thys. It seems probable that, as I have already elsewhere shown, the attached form 
of the medusa or the hydroid is a secondary condition, and that the primary con- 
dition is a direct development from the egg to the adult medusa. I would regard 
the ancestral form of metagenesis to be the development of the “ primitive me- 
dusa,” from an organism with both hydroid and medusan affinities, directly from the 
egg without attachment. From that medusa, — which I would call and have else- 
where named the primitive medusa,—in some instances, free medusiform gono- 
phores bud, as in Agalma; in other cases, the primitive medusa becomes attached, 
and is modified into a hydroid from which free gonophores separate ; while in still 
other cases, Nanomia, the primitive medusa is neither medusiform nor attached 
hydroid-like, but planula-like, with a float. The primitive medusa is homologous 
in all these changed forms. 
+ There is no reason to suppose that non-tentaculated genera allied to Hydra 
