er 
( 639 ) 
fact that the gasterozooids of the hydroid are devoid of organs for 
catching food. These facts at once suggest that the Ptilosarcus bene- 
fits the hydroid by helping it to secure food. 
On the other hand, on looking at a preparation of the hydroid, 
one is struck by the great number of dactylozooids which are so 
well provided with large nematocysts. Such a protection as these are 
capable of affording is probably more than is required by the small 
hydroid. The Jarge projecting spicules of the autozooids of the Penna- 
tulid also, would protect the hydroid. Therefore I would suggest 
that the Ptilocodium is of use to the Pennatulid in the warding off 
of enemies and in stinging prey by means of its batteries of large 
nematocysts and that the Ptilosarens by means of its projecting 
spicules, protects the Hydroid. Thus the Ptilosareus and Ptilocodium 
are mutually benefited. : 
Systematic position. 
Ptilocodium seems to have some affinities with Hydractinia, Podo- 
coryne and Millepora, as shewn by the sheet-like, encrusting basal 
coenosare from which the zooids arise independently ; but even in 
this character Ptiloeodium stands alone in having no chitinous or 
caleareous skeleton to protect it. 
In other respects Ptiloeodium is unique. The dimorphism of Ptilo- 
codium is quite distinct from that of Hydractinia and Millepora, and 
probably originated independently. In the first place, the gasterozooids 
of Ptilocodium are extremely unlike those of Hydractinia and Mille- 
pora. They are short, sessile, sac-like structures, without tentacles. 
Those of Hydractinia are long, filiform structures, provided with a 
crown of tentacles. The gasterozooids of Millepora are also much 
longer than broad, not at all sac-like in form, and are provided with 
knob-like tentacles. 
The dactylozooids, also, are very different in structure, in the three 
genera. Those of Ptilocodium are short and broad, and are furnished with 
four characteristic capitate tentacles. The endoderm of the body of the 
zooid and of the tentacles is solid and scalariform, giving the dactylo- 
zooids a marked appearance. The dactylozooids of Hydractinia are long 
and slender and very muscular so that they are capable of coiling and 
uncoiling themselves. They have also a central cavity and according 
to Miss Corcurr *) are provided with a terminal mouth. The dactylo- 
zooids of Millepora are very similar to those of Hydractinia. In 
possessing a solid scalariform endoderm, Ptilocodium resembles the 
Stylasterina. All the genera,except one, of this group, have the 
endoderm of the dactylozooids solid. 
1) Quart. Journ. Micro. Sci. No. 157, 1897. 
