t>34 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 ccerulea 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. 



E N. Endodermal layer. 



C. Homogeneous connective tissue layer. 



D. Layer of connective tissue cells. 



B, Very finely fibrous mass from winch 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 (0) 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 ccenenchymal tubes and also laterally with one another. These smaller 

 canals having usually a vertical course. Their summits are seen in Plate 

 I. fig. 5. 



