THE HYDROIDS OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 



83 



Sertularia rathbuni, Nutting, 1904. 



The specimens occur plentifully on a seaweed (Codium, sp.), standing erect and 

 rigid on its surface, so that it appears as if covered with a sparse coating of delicate 

 hairs. They are simple, altogether without branches, herein differing, but immaterially, 

 from the specimens described by Allman (1877), Versluys (1899), and Nutting, some 

 of which bore pinnules, and they attain a height of only 10 mm. The structures of the 

 stem are as previously described. The hydrothecae are in opposite pairs, the individuals 

 of which, in the distal part of the colony, are contingent for almost half their height, 

 but the line of contact gradually decreases until in the proximal pair the individuals 



Fig. 5. — Sertularia rathbuni. (a) Anterior aspect of stem, x 50. (b) Three-quarters view of hydrotheca showing 

 three- flapped operculum, x 60. (c) Gonangium. x 70. 



may be quite apart. Behind the stem the hydrothecae are always separate. Beneath 

 each hydrotheca, at the corner where base and inner wall meet, are two chitinous 

 processes which project downwards and lie alongside the wall of the internode. These 

 processes are more distinct in the older hydrothecae. The margin of a hydrotheca is 

 cut into three teeth, the lateral pair longer than the single superior tooth. There are 

 three opercular flaps. 



Gonosome. — The gonangia, which have not previously been described, occur on 

 many of the colonies. They are borne on the stem, from one to three in number, and 

 arise immediately beneath hydrothecae towards the base of the colony. In the speci- 

 mens which I have examined only one gonangium is apportioned to each hydrotheca- 

 pair. The gonangia are broadly ovate, marked with about six rather indefinite annula- 

 tions. Proximally they taper into a short stalk, while distally they contract into a 

 short wide neck, ornamented at its base by a circlet of bright spots, thickenings of the 



