188 



reference to other plastic colonies whose contour is subject to great modifica- 

 tion through position, the agency of curi'ents and the like. It leads one to 

 recognise that the general shape of a colony affords but a slender basis on 

 which to raise a superstructure of classification. 



The axis (fig. 1) consists of white calcareous internodes and brown horny 

 nodes. The internodes are symmetrically sculptured, ridges and furrows alter- 

 nating around the whole circumference. The ridges appear smooth to the 

 naked eye, but when slightly magnified present a beautifully serrated edge. 

 They vary in number in the different parts of the colony, there being fewer 

 in the younger portions. Twelve, thirteen and fourteen are common on the 

 larger branches. In the older parts of the colony the internodes are almost 

 cylindrical, but on the primary and secondary branches they are slightly 

 narrower at the middle, while in the twigs they are torpedo-shaped. The nodes 

 resemble fish-vertebrai in form. They are short and markedly constricted, and 

 have a silky lustre. At the ends they are slightly ridged, conforming to the 

 contour of the internodes ; but this gradually diminishes, so that at the centre 

 they are perfectly smooth. They contain no calcareous matter, except near the 

 base, where a central limy rod connects them with the internodes. In mass 

 they appear dark brown, but in section the colour is golden-yellow. 



The nodes and internodes vary in length throughout the colonies so that no 

 general size can be stated as typical. The following measurements of a node 

 and its adjacent internode were taken at different parts : — 



(a) Main stem : — 



Internode 4*5 mm. in length and 7 mm. in diameter. 

 Node 3 „ „ 6 „ 



{/)) Branch ; — 



Internode 6 mm. in length and 3 nuu. in diameter. 

 Node 6 „ „ 1-5 „ 



The branches arise from the calcareous internodes, generally one fi'om each, 

 but as many as three sometimes occur on one internode. In most cases 

 there is a considerable calcareous portion before the first node ; but this is often 

 so reduced that the branches seem to arise with a horny part. In other i)laces, 

 owing to the origin being close to the node, the branch seems to arise from it. 



Cross and longitudinal sections of a calcareous part, 1 cm. in diameter, 

 were ground to show the internal structure. There is a distinct radiate appear- 

 ance from centre to periphery. About 1'2 nmi. from the centre there is an un- 

 dulating line which corresponds to the grooves on the outside, but in this case 

 they are fewer in number. Towards the circumference, and at a distance 

 similar to the first, there is another almost identical line with a greater number 

 of undulations, but still fewer than those on the periphery. These wavy lines 

 doubtless correspond to what were previously the external surfaces of the axis. 



