MUSEUM OF COMPARATIVE ZOOLOGY. 223 
Calycophores. In 1883 the float was compared with the anterior nectocalyx 
of Diphyes. I regard the first nectocalyx of Monophyes as the same as the 
primitive hydrophyllium of Agalma.* 
In the phylogeny of the Siphonophores, both Physophores and Calycophores 
have a stage called the primitive larva in common, and it is possible their an- 
cestor was not unlike the so-called primitive larva stage of Agalma, with a 
single cap-shaped hydrophyllium, one polypite, and a knob similar to that 
which I have called the embryonic knob (Plate II. Fig. 8). The Calyco- 
phores retain certain organs which characterize this form, as the primitive 
hydrophyllium and the embryonic tentacular knob. In the Physophores, 
however, a specialized float, a modified bell, is developed, the embryonic knob 
gives place to other kinds of knobs, characteristic of the different genera of 
Physophores. : 
The primitive larva preserves the Medusa form, and may be supposed to 
approach more closely the ancestral form of the Siphonophores among other 
Hydromeduse than any other medusiform larva. The name primitive larva 
appears to me to be a good one, and can well be accepted as a help in studies 
of the phylogeny of the Siphonophores. 
I have already elsewhere devoted some space to showing that the primitive 
larva finds its nearest homologue among Hydromedusze, in certain Tubularian 
medusiform gonophores. I chose Lizzia for my comparisons. It was perhaps 
premature for me to take any one genus for such a comparison, and it might 
have been better to have chosen the Trachymedusz instead of the Tubularians 
for such an homology. From the character of the egg cleavage, and the fact 
that the development, as far as known, is similar. in the Siphonophores and 
genera of Trachymeduse, it is possible that the young stage of the former, 
which I have named the primitive larva, is more closely related to certain 
medusiform larve among the Trachymeduse than to genera like Lizzia. The 
close homology between a medusiform gonophore and a simple hydroid, how- 
ever, is such that I think we are justified in regarding the young of Nanomia 
with a float and no primitive hydrophyllium as homologous with the prim- 
itive larva of Agalma. I believe the ancestral form of all Hydromedusz, as 
well as of all the Siphonophora, will be found to be similar to the primitive 
larva of Agalma in its younger stages. It had the form of a ciliated planula, 
with an enlargement at one end and a mouth at the opposite. The enlarge- 
ment at one end was formed of three layers, is bell-shaped or gelatinous, and 
forms the bell of the Medusa, the float of Nanomia, and the primitive hydro- 
phyllium of Agalma. In the fixed hydroid it becomes a base of attachment, 
in Rhizophysa or Nanomia a float, and in Agalma a covering scale. It is 
well to have some name to designate this prototype, and no one has suggested 
any better one than the “ primitive medusa.” + 
* Embryological Monographs, No. III. Plate VII., Mem. Mus. Comp. Zéol., 
Vol. LX. No. 3. 
+ It would be interesting to trace the resemblance of this primitive larva or 
