230 THE MICROSCOPE. 



Dr. Grant estimates that a single Flustran has as many as four 

 hundred millions of these useful and restless appendages. The feelers 

 vary from ten to twelve ; their organisation consists of a long gullet, a 

 gizzard, a stomach and intestines; the body being of a transparent 

 substance. Some take the form of a delicate minute tree, having cells 

 in all parts; and vary in colour, inhabiting every sea. Lamouroux 

 says : " When the animal has acquired its full growth, it flings from 

 the opening of its cell a small globular body, which fixes near the 

 aperture, increases in size, and soon assumes the form of a new cell ; 

 it is yet closed, but through the transparent membrane that covers its 

 surface the motions of the polyp may be detected ; the habitation at 

 length bursts, and the tentacles protrude ; eddies are produced in the 

 water, and conduct to the polyp the atoms necessary for its sub- 

 sistence." 



Dr. Grant writes : " The aperture of the cells is formed by a semi- 

 circular lid, convex externally and concave internally, which folds down 

 when the polyp is about to advance from the cell. The opening of this 

 lid in the F. truncata, where it is very long, appears through the 

 microscope like the opening of a snake's jaws ; and the organs by which 

 this motion is effected are not perceptible. The lids of the cells open 

 and shut in the Flustrae without the slightest perceptible synchronous 

 motion of the polyps." 



Milne Edwards, in writing on the Eschares, remarks, " that the cell, 

 in which it is said the polyp retires as into a shell, is a component part 

 of the animal itself, in which it conceals itself, if we may use the com- 

 parison, as the hedgehog enters into the thorny skin of his back. It is 

 not a calcareous crust, which is moulded on the surface of its body, 

 but a portion of the general tegumental membrane of the skin of the 

 polyp, which, by a molecular deposit of earthy matter in the meshes of 

 its tissue, ossifies as the cartilages of superior animals ossify, without 

 ceasing to be the seat of the nutritive movement." 



In the formation of their stony skeletons, the animals appear to 

 take no part, " being secreted by the integuments or membranes with 

 which it was permeated and invested, in like manner as the bones 

 and nails in man are secreted by the tissues designed for that purpose, 

 and acting without his knowledge or control. From an analysis of 

 the stony corals, it appears that their composition is very analogous to 

 that of shells. The porcellaneous shells, as the cowry, are composed of 

 animal gluten and carbonate of lime, and resemble, in their mode of 

 formation, the enamel of the teeth ; whereas the pearly shells, as the 

 oyster, are formed of carbonate of lime and a gelatinous or cartila- 



