REPORT ON CORALS— EXPLANATION OF PLATES. 235 



Fig. 2. Transverse section through the uppermost part of a retracted polyp of HeJioporn 

 ccerulea, as viewed from below, showing the under surface of the most super- 

 ficial structures closing the mouth of the calicle, i.e., the immediate under 

 surface of the polyp-lobes seen in Plate I. fig. 5, and Plate II. fig. 1. The 

 drawing is from a decalcified preparation. The soft parts lining the wall of 

 the calcareous calicle are cut through ; they retain the form of the calicle, 

 to which they were closely applied. The wall presents a series of longi- 

 tudinal folds so as on transverse section to show a sinuous outline with 

 twelve indentations separated by twelve bulgings. The indentations occupied 

 in the fresh condition of the animal by calcareous matter represent the 

 twelve ridge-like calcareous septa present in the calicle. The indentations 

 are neither in form nor arrangement symmetrical, nor are the eight mesen- 

 teries (MM) arranged symmetrically with regard to them. 



Between the mesenteries the body-wall of the polyp does not reach out- 

 wards eveiywdiere the entire distance to the wall of the calicle, but is con- 

 tinuous with this only in the region of its indentations. At each of the 

 bulgings of the wall a wfide aperture is left, by which the cavity of the polyp 

 communicates with the canal systems around. 



MM. Mesenteries. 



0. Openings by which the polyp cavity communicates with the canal- 

 systems. The light oval spaces shown in the shaded areas of the 

 openings are the sinus of the superficial canal-system. 

 Fig. 3. Vertical section through one of the siphonozooids of Sarcophyton. On the left- 

 hand side of the drawing the calcareous spicules are shown in situ. On the 

 right the appearance presented after these have been removed by acid is shown. 



The points of the spicules accompanied by a layer of connective tissue 

 project up far into the prominent layer and raise it up just as do the external 

 ectodermal points of hard tissue in Heliopora. The connective tissue shows 

 excessively small ramified nuclei scattered through its otherwise homogeneous 

 tissue. Portions of adjacent siphonozooid cavities are shown on either side 

 of the central one ; the transverse fibrillation of their wall is indicated. 



Sp. Spicules. 



S.C. Cavities from which spicules have been removed by acid. 



S. Stomach of the siphonozooid provided within with cilia directed inwards. 



M E. Its mesenteries. 



M F. Mesenterial filaments. 



C. Canal of the transverse system, forming a communication between two 

 adjacent siphonozooid cavities and lined by endodermal cells con- 

 tinuous with the layers lining the siphonozooid cavities. 



