292 Proceedings of the Royal Irish Academy. 



Coenenchyma. — Figs. 10 {a, b, and c) and 13 («, 6, and c) are given to show 

 the structvu'e of the coenenehyma at the same levels as the corresponding 

 figs. 9 («, b, and e) and 12 (a, b, and c) respectively. The coenenehyma may 

 be differentiated into two distinct regions — (1) an outer superficial layer, in 

 which the polyps are embedded, and which contains no canals ; and (2) an 

 inner layer, which is intersected by a mesh work of transverse canals. 



In var. a the superficial layer is much thinner than the inner layer ; but 

 in var. b the superficial layer is the thicker of the two. At the tip of the 

 colony there is a conical growing point devoid of polyps and having no canals. 

 The thickness of the coenenehyma hardly varies throughout the colony. 



Canal Si/sfems. — Tliis consists of (1) a longitudinal system and (2) a 

 transverse system. The longitudinal system is composed of two series of 

 longitudinal canals — (a) an outer, situated between the two layers of the 

 coenenehyma, and with which the polyps connect directly ; and (b) an inner, 

 situated between the inner layer of the coenenehyma and the axis. These 

 two 8y8t«ms are united by the transverse canals which penetrate the inner 

 layer of the coenench)Tna. 



The number of canals in each of the two longitudinal series diminishes in 

 number from the base of the colony upwards. This, of course, is natural, 

 since the number of polyps also decreases. 



In the inner series of longitudinal canals there are two, situated diametri- 

 cally opposite one another, which are much larger than the others. They 

 occur in both varieties ; but in the case of var. b they correspond to the 

 position of the longitudinal bare spaces. 



The greater thickness of the coenenehyma in var. a may explain the 

 absence of this phenomenon in the latter variety. 



ArU. — The axu< is marked by longitudinal ridges and furrows. The 

 number of these correspond to the number of canals in the inner longitudinal 

 series, and consequently diminishes towards the apex, so that this character 

 is of no taxonomic importance (fig. 11 (a, b, andc)). The structure of the 

 axis is ver)' well seen in this species. It is composed of concentric laminae 

 of horn, impregnated with small limy sclerites. To the inside of the canals 

 of the inner longitudinal series a layer may be detached showing the sclerites 

 in situ. The innermost layers are much more densely spiculose than the 

 outer. The diameter of the axis (unlike the coenenehyma) gradually 

 diminishes towards the tip of the colony, where it becomes almost hair-like, 

 and contains very little lime. 



