STUUCTUlllO or IMf.S JItlM'UllIS. 429 



tendency to lie horizontally, the colony would produce branches 

 towards the upwardly directed surface. It seems to me that this 

 consideration is worthy of attention in reference to other plastic 

 colonies whose contour is subject to great modification tlirough 

 position, the agency of currents, and the like. It leads one to 

 recognize that the general shape of a colony affords but a slender 

 basis on which to raise a superstructure of classification. 



In this specimen also the development of club-shaped ends on 

 the twigs is hardly noticeable ; and an this feature is most pre- 

 dominant in the forms with developing eggs, it may be a nutritive 

 phenomenon or a natural safeguard against premature rupture by 

 the ever-increasing abnormally large embryos. 



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

 brown horny nodes. The internodes are symmetrically sculp- 

 tured, ridges and furrows alternating around the whole circum- 

 ference. The ridges appear smooth to the naked eye, but when 

 slightly magnified present a beautifully serrated edge. They 

 vary in number in the difl'erent parts of the colony, being com- 

 paratively 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 cyliodrical, but on 

 the primary and secondary branches they are slightly narrower 

 at the middle, while in the twigs they are torpedo-shaped. Tfie 

 nodes resemble fish-vertebrse in form. They ai'o 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 neur 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 interJiodes vary in length throngliout 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 milliras. in length and 7 millims. in diameter. 

 Node 3 „ „ 6 „ „ 



(b) Branch : 



Internode G millims. in length and 3 millims. in diameter. 

 Node „ „ „ l-r> ,, „ 



TiTNN. .TOURN. — ZOOLOUY, VOL. XXIX. 31 



