MUSEUM OF COMPARATIVE ZOOLOGY. 223 



Calycophores. In 1883 the float was compared with the anterior nectoealyx 

 of Uipliyes. I regard the first nectoealyx 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 diff"erent 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 

 Hydromedusee 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 Hydromedusse, 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 Trachymedusse 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 Trachymedusse, it is possible that the young stage of the former, 

 which I have named the primitive larva, is more closely related to certain 

 medusiform larvae among the Trachymedusse 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 Hydromedusse, 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 i^rimitive hydro- 

 phyllium of Agalma. In the fi.xed hydroid it becomes a base of attachment, 

 in Pihizophysa 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." f 



* Embryological Monographs, No. III. Plate VII., Mem. Mus. Comp. Zool., 

 Vol. IX. No. 3. 



f It would be interesting to trace the resemblance of this primitive larva or 



