REPORT ON CORALS — EXPLANATION OF PLATES. 933 



Z. Autozooids. 



P M. Protractor muscle. 



E M. Retractor muscle. 



C. One of the canals of the transverse system. 



Fig. 3. Diagrammatic representation of a transverse section across a polyp of Heliopora 

 ccerulea decalcified, taken just below the mouth. The tips of four of the 

 tentacles cut across are retained in situ; the other four tips had fallen out in 

 the section from which the drawing was constructed. 



D. External layer of connective tissue cells. 



C. Layer of homogeneous connective tissue giving off a series of stout 



offsets (A), which pierce the layer (D) and project externally. Con- 

 tinuations of this same layer are seen to form the central plate of the 

 mesenteries and a wall around the stomach. 



E N. Endoderm lining the whole of the intermesenteric chambers. 



M. Mesenteries. 



R M. Retractor muscles. The " Dorsalfach " of Kolliker lies below in the 

 drawing, the " Ventralfach " above. The muscles are covered by the 

 endoderm. 



T T. Tentacles cut across. 



S. Stomach. B. Its muscular layer. 

 Fig. 4. Section parallel to the surface of Heliopora ccerulea, to show the relation of the 

 hard parts to the soft. 



T C, T C and the similar oval spaces represent the tubular cavities of the 

 ccenenchym cut across. The interspaces between these cavities are 

 occupied by the hard tissue. 



The hard tissue is fibrous in structure, the fibres radiating from 

 a series of axes here seen cut across (C). Suture-like lines (S) occur 

 occasionally where the peripheries of the various radiating systems 

 join. 



D. Layer of connective tissue cells. 



C. Layer of homogeneous connective tissue. 



E N. Endodermal layer. 

 Fig. 5. Polyp of Heliopora ccerulea and the immediately surrounding structures as 

 viewed in a thick horizontal section from the outer surface. 



The external ectodermal layer is not seen, being too transparent. The 

 superficial projecting points of the hard tissue of the corallum being opaque, 

 are shown shaded dark. Away from the calicle the points are arranged in 

 parallel rows. All over the surface are seen the tops of the ramifications of 

 the superficial system of canals or sinus (fig. 1, V, V); those immediately 



(ZOOL. CHALL. EXP. — PART VII. — 1880.) G 30 



