HYDROID ZOOPHYTES. 27 



plan of regarding it as sjnonymous with " sarcotheca " and applicable only to 

 perisarcal skeleton. The zooid which the nematophore envelopes (in the case of 

 Halecium only very partially) appears to us to be a true dactylozooid, and we have 

 not adopted the use of any of the terms " sarcostyle," " machopolyp," " sarcodal 

 process," etc., suggested by various wri.ters. 



The occurrence of the gonothecse in " coppinia " masses in our specimens of 

 Halecium arboreum is a feature of some interest. The number of genera in which this 

 grouping of the sexual zooids occurs is extending as our knowledge advances. 



The gonothecse of the specimens of Halecium arboreum. and //. telescojnciim 

 obtained by the ' Challenger ' were not observed, but the gonothec^ of //. flexile, 

 H. dichotomum, H. fastigeatum, H. beanii and H. cymiforme obtained by the same 

 expedition were not in " coppinia " masses. This peculiar grouping of the gonothecse 

 is not, therefore, a character of the genus, but may indicate a method for the future 

 rearrangement of the species into subgeneric groups. 



Halecium arboreum. 



ft 



(Plate IV., figs. 27, 28, 29.) 

 Halecium arloreum, AUman, 'Challenger' Keports, Vol. XXIII. (1888), pi. iv., figs. 1-3. 



Localities. — The species is evidently abundant in McMurdo Bay, extending from 

 shallow water to depths of 130 fms. 



"W.Q., McMurdo Bay, Febi'uary 20th and 28th, 1902;- 20 fms. W.Q., Flagon 

 Point, January 17th and February 23rd, 1903; 10-20 fms. W.Q., No. 6 hole, 

 January 31st, 1903; 130 fms. W.Q., February 21st, 1902; 10 fms. W.Q., off 

 cable, February 17th, 1904. East End of Barrier, January 29th, 1902; 100 fms. 



The specimens of this species consist of a number of robust colonies rising to a 

 height of 300 mm. from a thick hydrorhizal plexus. A few small colonies from the 

 east end of the Ice Barrier were found attached to the stems of Plumularia. 



Hydrosome. — The hydrorhiza is a dense plexus of tubes exhibiting a tendency to 

 be grouped together in polysiphonic bundles. 



The main stem is a thick polysiphonic hydrocaulus 10 mm. in diameter. 



The proximal branches are polysiphonic, like the main stem, and anastomose freely 

 whenever they come in contact with one another. The distal branches are much more 

 numerous and usually monosiphonic. The monosiphonic branches (fig. 27) alone bear 

 the hydrotheese, but in some cases these branches still bear hydrothecse after the 

 additi*! of the first few strengthening tubes. 



The ramification of the distal branches is pinnate, and in other parts of the colony 

 it is roughly, but not rigidly, flabellate. 



The thickness and roughness of the stem and of the principal branches afford 

 admirable support for the hydrorhizse of other hydroids, a list of which will be found 

 on page 1. 



2 F 2 



