RB} THE VOYAGE OF H.M.S. CHALLENGER. 
around the polyp-lobes have a radiate arrangement. These canal cavities 
are lined with endodermal cells. 
In the centre of the drawing are seen the eight lobes of the contracted 
polyp with the mouth of the atrium in their centre. In their peripheral 
region the endodermal lining of the cavities of these lobes is seen showing 
through their superficial tissue. 
Each of the lobes further shows a fine longitudinal striation, probably 
caused by muscular fibres, and some very small nuclei at its inner aspect. 
P. Projecting points of the corallum. 
A A. Sinus of the superficial canal-system. 
L. Lobes of contracted polyp. 
Fig. 6. Portion of a section of Heliopora caerulea cut parallel to the surface and viewed 
from beneath. From a specimen decalcified in chromic acid. The drawing 
represents a spot where a growing point of the corallum has been removed 
by the acid. 
EN. Endodermal layer. 
C. Homogeneous connective tissue layer. 
D. Layer of connective tissue cells. 
B. Very finely fibrous mass from which the calcareous tissue has been 
removed. 
A. Cavity in the mass, 
PLATE II. 
Fig. 1. Diagram to show the canal-systems in connection with the summits of the 
ccenenchymal tubes and calicles. 
A. Cavity of a calicle. 
B. Cavity of a tube. 
D. Walls of the tubes and calicles in longitudinal section. 
The channels of the deep canal-system (V, V’) lead over the summits of 
the walls from one tube to another, and from the tubes to the calicular 
cavities. The canal (V’) opening into the calicular cavity in the figure 
corresponds to the opening (QO) in fig. 2. 
C. A similar canal leading into a tube behind B. 
Above in the figure is seen the superficial canal-system, consisting of 
smaller canals and sinus communicating with the deep canals and roofs of 
the ceenenchymal tubes and also laterally with one another. These smaller 
canals having usually a vertical course. Their summits are seen in Plate 
tie, 5, 
