ZOOPHYTES. 273 



is secreted by them, yet, when once formed, it has 

 no further power of development, nor is any circu- 

 lation carried through it. But in the moss corals, 

 the outward skin, the polypidom, continues to be 

 always a living part of the animal which dwells 

 within; it adheres closely to the polype, and is 

 connected with its flesh. None of the animals of 

 this class are separate or naked, like the sea ane- 

 mones; they all grow in compound bodies, and 

 are lodged in cells, in which, during a state of 

 repose, the polypes lie doubled up. The species 

 of moss corals are very numerous. Hundreds 

 of fossil kinds, undiscovered but by the aid of 

 most powerful microscopes, have recently been de- 

 tected by Ehrenberg and D'Orbigny, their shelly 

 covering entering into the composition of chalk 

 beds, mountain limestone, and the flints of the Jura 

 limestone, sea sands, and the sands of the Libyan 

 desert. Many of these are altogether invisible to 

 the unassisted eye; others appear like minute 

 points, smaller than the grains of the sandy shore. 

 So minute are some of the species of moss corals, 

 that great accumulations of them are found in the 

 finest prepared whiting, uninjured in form, amid 

 all the processes of its preparation. And when we 

 call in the aid of the microscope we may behold 

 them of exquisite symmetry, in the chalk coating 

 which covers the walls of our apartments, forming- 

 there a beautiful mosaic work when the chalk, 

 mixed with water, is spread out before us. 



A large number of the moss corals appear to us 

 merely as scaly crusts upon the surfaces of the 

 rocks and sea-weeds, often covering them with a 

 thick coat of dirty sand colour, and, as those un- 

 acquainted with zoophytes would think, spoiling 



