BKANCHIOCERIANTHÜS IMPERATOR (aLLMAN). 249 



In Other hydroids these two elements are borne on separate stalks, 

 as for example in Pennalia. 



Hydrocaulus. 



The under part of the hydranth is prolonged to a shallow 

 funnel whose neck corresponds to the hydrocaulus. At about 

 the point where the hydranth joins the hydrocaulus, there is 

 a circular constriction (Woodcuts 1 & 2). Here the diameter of 

 the hydrocaulus is only 9 mm. and from this part down to the 

 base it increases in its diameter. Within the constriction is a 

 diaphragm (Woodcut 1, I, II, m') separating incompletely the 

 cavity of the hydranth from that of the hydrocaulus. In other 

 words the circular constriction is the surface expression of the 

 insertion of the diaphragm. In the midst of this partition there 

 is an opening (Woodcut 1, I, II, C) which puts the two cavities 

 above and below in communication. It is about 4 mm. in dia- 

 meter and is almost circulai*. The plane of the diaphragm is not 

 visibly oblique to the long axis of the hydrocaulus. In the speci- 

 mens of 3Ionocaidus imperator in the British Museum, this dia- 

 phragm is, according to Dr. Mark, distinctly oblique and the 

 central opening is much elongated. 



The hydrocaulus is a hollow tube which has a total length 

 of G50 mm. including the proximal end with hair-like appen- 

 dages. The hydrocaulus, even when fresh, was collapsed and 

 more or less longitudinally folded, so that the exact measure- 

 ment of its diameter was almost impossible. Approximately, it 

 was 15 mm. just below the constriction, 25 mm. at the middle, 

 and 42 mm. at the terminal root. 



The outer surface of the hydrocaulus is smooth. In the upper 



