A cylindrical tube connected with the mouth represents the oesophagus or gullet; but all other portions of the digestive tube are very rudimentary. The oesophagus connects the general cavity of the body with the exterior, and looks as if it were suspended in the middle of the body by certain folds, which issue with perfect symmetry from eight points of its circumference. The folds which thus fix the oesophagus form a series of cells, above each of which it attaches itself, and supports an arm or tentaculum. [Illustration: Fig. 50. Another form of the Coral Polyp. (Lacaze-Duthiers.)] Let us pause an instant over the soft and fleshy bark in which the polyps are engaged. Let us see also what are the mutual relations which exist between the several inhabitants of one of these colonies, how they are attached to one another, and what is their connection with the polypidom. The thick fleshy body, soft, and easily impressed with the finger, is the living part which produces the coral; it extends itself so as exactly to cover the whole polypidom. If it perishes at any one point, that part of the axis which corresponds with the point no longer shows any increase. An intimate relation, therefore, exists between the bark and the polypidom. If the bark is examined more closely, three principal elements are recognized--a common general _tissue_, some _spicula_, and certain _vessels_. The general tissue is transparent, glossy, cellular, and contractile. [Illustration: Fig. 51. Coralline Spicula. (Lacaze-Duthiers.)] The _spiculæ_ are very small calcareous concretions, more or less elongated, covered with knotted joints bristling with spines, and of regular determinate form (Fig. 51). They refract the light very vividly, and their colour is that of the coral, but much weaker, in consequence of their want of thickness. They are uniformly distributed throughout the bark, and give to the coral the fine colour which generally characterises it. The vessels constitute a network, which extends and repeats itself in the thickness of the crust. These vessels are of two kinds (Fig. 52); the one, comparatively very large, is imbedded in the axis, and disposed in parallel layers; the others are regular and much smaller. They form a network of unequal meshes, which occupies the whole thickness of the external crust. This network has direct and important connection with the polyps on the one hand, and with the central substance which forms the axis on the other. It communicates directly with the general cavity of the body of the animal, by every channel which approaches it, while the two ranges of network approach each other by a great number of anastomosing processes. Such is the vascular arrangement of the coral. [Illustration: Fig. 52. Circulating Apparatus for the nutritive fluids in the Coral. (Lacaze-Duthiers.)]