( 640 ) 
In all other points except the dimorphism, however, Ptilocodium 
differs from the Stylasterina, and on the importance of dimorphism 
as indicating close relationship too much stress should not be laid. 
The old group Hydrocorallinae affords an illustration of the result 
of laying too much stress on this factor. In this group, the Mille- 
porina and the Stylasterina were formerly united, but it has since 
been pointed out that differences of more importance necessitate their 
being classed as separate orders. 
The next point to consider in discussing tbe relationship between 
Ptilocodium, Hydractinia and Millepora, is that of the gonophores. 
Ptilocodium resembles Hydractinia in having adelocodonic gonophores 
but differs from it in having the gonophores arising from the base 
of the ordinary gasterozooid and not from a specialised individual or 
blastostyle. In Millepora there is no blastostyle and the medusae 
arise independently of the gasterozooids, from the surface of the colony. 
In Perigonimus, which, in the important respect of the structure 
of the basal coenosare does not closely resemble Ptilocodium or Hy- 
dractinia, the gonophores arise from the hydrocaulus bearing the 
gasterozooid or from the hydrorhiza. This case may therefore be taken 
as an indication that too much stress must not be placed on the 
position of origin of the gonophore in the colony. 
Ptilocodium stands quite apart from other genera of Hydroids with 
epizoic habits. 
The epizoic Hydroid Stylactis minoi described by Arcock *) was found 
on the fish Minous inermis, but differs considerably from Ptilocodium. It 
has bydranths crowned with numerous tentacles rising from the 
hydrorhiza. It is not provided with dactylozooids of any kind and 
its gonophores are in the form of sporosacs arising from specialised 
individuals which bear tentacles. . 
Ptiloeodium also differs considerably from Hydrichthys mirus, a 
Hydroid described by Fewkes?) as epizoic on the fish Seriola zonata. 
This hydroid exhibits structures of two kinds arising from the basal 
plate. In the first place there are long, filiform hydranths, which 
are looked upon as degenerate gasterozooids and secondly clusters 
of botryoidal gonosomes. Dactylozooids do not occur. 
Ptilocodium has obviously no affinities with the hydroid Nudiclava 
described by Lroyp®), nor with Moerisia lyonsi, a Hydromedusan 
from Lake Qurun, described by C. L BOULENGER *). 
1) Arcock A. Ann. & Mag. Nat. Hist. 1892 vol. X p. 207. 
2) Fewkes. Bull. Mus. Comp. Zoöl. vol. XIII p. 224. 
3) Lroyp R. E. Records of Indian Museum. Vol. I. Part IV. 
4) Boutencer C. L. Quart. Journ. Micros. Sci. Vol. lij Jan. 1908. 
