STRUCTURE AND RELATIONS OE CERTAIN CORALS. 
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tubes and laterally with one another. They have usually a vertical course. 
Their summits are seen in Plate 8. fig. 5. 
Fig. 8. Transverse section through the uppermost part of a retracted polyp of lleliopora 
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 8. fig. 5 and Plate 9. fig. 7. 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 mesenteries 
(MM) arranged symmetrically with regard to them. 
Between the mesenteries the body-wall of the polyp does not reach out- 
wards everywhere 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 wide aperture is left, by which the cavity of the polyp 
communicates with the canal-systems around. 
M M. Mesenteries. 
O O. 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. 9. Vertical section through one of the zooids of Sarcojphyton. 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 epithelial layer, raising it up just as the points of hard 
tissue in Heliopora. The connective-tissue shows excessively small ramified 
cells scattered through its otherwise homogeneous tissue. Portions of 
adjacent zooid-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 zooid provided within with long cilia directed inwards. 
ME. Its mesenteries. 
MF. Mesenterial filaments. 
C. Canal of the transverse system, forming a communication between 
two adjacent zooid-cavities and lined by entodermic cells continuous 
with the layers lining the zooid-cavities. 
