522 THE MICROSCOPE 



the polype 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 

 Flustrce without the slightest perceptible synchronous 

 motion of the polypes." 



In the formation of their stony skeletons, the animals 

 appear to take a most insignificant part ; they are princi- 

 pally secreted by the integuments or membranes with which 

 they are invested, in like manner as the bones and nails in 

 man are secreted by tissues designed for that purpose, and 

 acting slowly and imperceptibly. From an analysis of the 

 stony corals, it appears that their composition is very 

 analogous to that of shells. The porcellaneous shells, as 

 «,he 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 

 cartilaginous substance, the earthy matter being secreted 

 and deposited in the interstices of a cellular tissue, as in 

 bones. In like manner, some corals yield gelatine upon 

 the removal of the lime, while others afford a substance in 

 every respect resembling the membranous structure ob- 

 tained by an analysis of the nacreous (pearly) shells. 

 A recent elaborate analysis of between thirty and forty 

 species of corals, by an eminent American chemist (Mr. B. 

 Silliman), has shown, contrary to expectation, that they 

 contain a much larger proportion of fluorine than of 

 phosphoric acid. 



F lustra foliacea, the broad-leaved Horn-wrack of Ellis, 

 is about four inches high, and of a brown colour. Tlie cells 

 are small, in alternating rows ; and sometimes covered by 

 a lid opening downwards. Hook says : " For curiosity and 

 beauty, I have not, among all the plants or vegetables 

 I have yet observed, seen any one comparable to tliis 

 sea-wccd." FLustra truncata is abundant in deep water, 

 and grows to a height of about four inches ; it is of a 

 delicate yellow colour, and bushy. This is the narrow- 

 leaved Horn-wrack of Ellis ; for it must not be forgotten 

 that the older writers regarded the whole genera as plants. 



