10 SYDNEY J. HICKSON and F. H. GRAVELY. 



It is necessary in the description of this very remarkable hydroid to refer 

 constantly to the special characters of the two specimens we received. We shall 

 therefore call them specimen A and specimen B respectively. 



Specimen A consists of a single continuous colony encrusting the stems of a 

 specimen of Halecium arbor eum. From this encrustiag mas§, which possesses all the 

 general features of an ordinary Hydractinia, a single upright branching stem arises, 

 which has some resemblance to the stems of Hydrodendrium gorgonoides (Nutting 20 : 

 pp. 936-938 ; PL I., figs. 1-6 ; PI. VIL, figs. 1-2). 



Specimen B consists of a large number of brittle and broken stems bearing 

 Hydractinian hydranths, but without any encrusting base or hydrorhiza. It seems 

 probable that all these broken stems belong to one colony, and we are convinced 

 that specimen B belongs to the same species as specimen A. 



Specimen A. — There are two regions in this specimen, the encrusting or basal 

 region, and the upright branching stem, the rhizocaulus. The basal part (fig. 9, 

 enc. r., fig. 9a) entirely surrounds the polysiphonic stems of Halecium arboreum, only 

 a few pinnules of the supporting hydroid penetrating it and being exposed. It is 

 about 0"4 mm. 'thick, the outer part to a depth of about 0*2 mm. from the surface 

 containing coenosarc, the lower part consisting of a lacunar skeleton with strands of 

 tissue in some of the lacunae, whilst others may be empty. The " spines " of other 

 species of Hydractinia and of Podocoryne are represented in this species by a series 

 of low ridges (fig. 7, r. sp.), often continuous with each other, but projecting 

 irregularly on .the surface of the colony. They have some resemblance to the 

 hydrophores of Ceratella (Spencer 21), but seem to have no definite relation to the 

 zooids. There are only two kinds of zooids, the gastrozooids and the blastostyles 

 (gonozooids). 



The gastrozooids (fig. 7, gz.) vary a good deal in length. The usual length is 

 about 2 mm., but they are in some cases as much as 4 mm. in length. There is a 

 conical hypostome surrounded by a single circlet of from 9 to 13 tentacles, each 

 about 0'6 mm. in length. It is evident that the gastrozooids are extremely 

 contractile, and it is noteworthy that in some cases they have been killed introverted, 

 the circlet of tentacles having assumed a position at the base. 



The blastostyles (fig. 7, hi.) are from 0*3-0 '7 mm. in height. They are small 

 and degenerate. They usually exhibit a few small tentacles 0-05 mm. in length, but 

 in some cases no tentacles at all could be seen. They may have a minute mouth, 

 but usually have not. The gonophores (fig. 7, gph.) are all probably female and 

 in the form of sporosacs. They occur in a single circlet round the base of the 

 blastostyle. 



The upright branching stem of this specimen rises to a height of about 40 mm., 

 and is about 0*8 mm. in diameter at the base (fig. 9,/r. rhc). The ramification is 

 irregular, the eight or nine terminal branches ending in some cases in a long 

 filamentous process. The surface of these branches is remarkably smooth. The 



