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Expedition were two specimens of Ptilosareus sinuosus (Gray)., and 
growing over the tips of the leaves of them a small epizoic hydroid 
was discovered. To a description of this new hydroid, the present 
paper is devoted. | 
The occurrence of an epizoite on a Pennatulid is in itself an 
interesting fact as the Pennatulids have usually been regarded as 
being peculiarly free from any such growths. Only two specimens 
of Ptilosarcus were received from the Expedition and the hydroid 
occurs on both of them. The other Pennatulids of the collection 
have heen carefully iooked over, but on none of them has an extra- 
neous growth of any kind been found. 
Ptilosarcus belongs to the Pennatuleae, the section of Pennatulids 
which are distinctly bilaterally symmetrical and have the autozooids 
in rows, with their body walls fused to form leaves. 
Along the free edges of the leaves of the given specimens of 
Ptilosarcus, the hydroid Ptilocodium grows (fig. 1). It is quite visible 
to the naked eye, though in a cursory glance over the leaves of the 
Pennatulid, it probably would not be noticed. The hydroid affects 
the free edges of all the leaves of the Pennatulid, even those at the 
free extremity. It is suggestive that it does not spread over the rachis 
of the Pennatulid nor even over the main surfaces of the leaves, 
but grows only over the oral ends of the autozooids. 
The hydroid is devoid of any kind of skeleton and spreads over 
the distal parts of the antozooids composing the leaves of the Ptilo- 
sarcus (figs. 2, 3). The colony grows by means of spreading stolons. 
These stolons run singly over the spicular projections of the autozooids 
and along their tentacles ; (figs. 2, 3, 4) but over the part immedia- 
tely below this, they branch and closely anastomose forming a more 
or less continuous sheet of basal coenosare. 
The hydroid exhibits the phenomenon of dimorphism, the gastero- 
zooids and dactylozooids being quite distinct. The zooids are sessile, 
arising directly from the stolon or basal coenosare as the case may be. 
The dactylozooids arise at very short intervals along the stolon 
and are far more numerous than the gasterozooids. Gonozooids occur 
at frequent intervals. They are much fewer. in number than the 
Note by Professor Hickson. The hydroid described in this paper was found on 
the only two specimens of Ptilosarcus in the Siboga collection. As they appeared 
to be of very great interest, and I could not part with the Pennatulids which are 
themselves under investigation, I considered it to be advisable, in the interests of 
science, that a description of them should be prepared in my laboratory without 
undue delay. [| wish to express my hearty thanks to M. Bittarp, to whom the 
description of the Hydroidea of the Expedition has been entrusted for kindly giving 
his sanction to the publication of this paper independently of his memoir. 
