J51MERIA KIGIDA SP. N. 6 



which tightly clasp the support, recalling certain root-climb- 

 ing plants (text-fig. 1). By these two means the hydrorhiza 

 becomes very firmly attached to the smooth surface of the weed 

 on which it is growing. The analogy to a climbing plant is 

 further emphasised by the fact that the hydrorhiza gives off 

 in an alternating manner hydrocaulus-outgrowths, recalling 

 petioles, each bearing a single hydranth at the distal end. 

 The root-like clasping organs often, although not invariably, 

 spring from the node, or the place of origin of the hydro- 

 caulus-OLitgrowth. It is interesting to note that the methods 

 of accomplishing similar mechanical feats (namely, (1) grasp- 

 ing a slippery surface, (2) exposing the hydranths of the 

 hydroid, or the leaves of the plant, to the surroundingmedium 

 in such a manner that they shall interfere with one another 

 as little as possible) are closely alike in two totally different 

 organisms. 



The hydrorhiza is smooth and is without annulations of any 

 kind. The diameter is about 0'095 mm. (95^) and the thick- 

 ness of the perisarc is 12-20 /i on the sides and upper surface. 

 As usual the thickness of the perisarc is much less on the 

 surface of attachment to the weed, and sometimes here it is 

 almost non-existent. Very frequently the sides of the perisarc 

 are stengthened by vertical arches or pillars some 30 fx in 

 thickness (PI. II, figs. 13, 14, and 15, jj.). The structure 

 is built for strength, and in cross-section of the hydrorhiza the 

 pei'isarc resembles the span of a bridge with thick, vertical 

 buttresses. 



The outer layer of the perisarc is apparently somewhat soft 

 and gelatinous in character, since diatoms and minute particles 

 of mud and sand adhere to it and form a dense fringe. 



Internally there is a simple coenosarc tube of endoderm 

 surrounded by ectoderm. There is no accessory thin ecto- 

 dermal sheet lying in contact with the roof of the perisarc- 

 tube as occurs in certain hydroids. 



Hydrocaulus. — The hydrocaulus consists of branches 

 arising from the hydrorhiza, each branch carrying a single 

 hydranth. The branches or hydranth-pedicels do not come 



