Zoophytes. 209 



tlie LaomedeoB it is very common to find branches torn off, but the 

 specimens show evident signs of great violence in the destruction of 

 their cells, and other marks of depauperization. The Sertularia nigra, 

 a remarkably stout and rigid species, which grows near the Eddystone 

 in the English Channel, is remarkably exposed to strong and con- 

 stantly changing currents, yet the shedding of the pinnae is compara- 

 tively rare in it, and the ovarian vesicles are constantly produced in 

 rich profusion. If the sheath were subject to no change after it was 

 once formed, the younger pinnae would suffer equally with the older 

 ones, which is not the case. When either the pinnae or vesicles dis- 

 appear, they leave behind well-defined cicatrices, which gradually 

 become nearly obliterated, and not jagged and irregular marks, as if 

 produced by violence. They appear to fall, after having performed 

 their functions, from a process very similar to that by which trees shed 

 their leaves and fruits. 



The sheath also, under certain conditions, affords signs of irritation 

 which could not be expected from a dead membrane. In the Cam- 

 panularia volubilis, in which the cells stand on long slender footstalks, 

 which are annulated at each extremity and plain in the centre, the 

 whole length of the peduncle will become corrugated, as if ringed 

 throughout. Ellis figures them in this condition at pi. 14, fig. A, in 

 his Essay on Corallines ; and I have seen them assume this appear- 

 ance, and again change it, while under the microscope : in either 

 state, and while the change was going on, the polype appeared to be 

 very little influenced by it, or to have any effect in producing it. The 

 extremities of the branches and trunk are always more tender than 

 other parts, from being more recently formed : these points frequently 

 give way, and the pulp is forced through the ruptured parts. This 

 extravasation is sometimes so extensive that in the knotted sea-thread 

 [Laomedea genie iilatd), sea-bristles [Plumularia setaeea), «&c., scarcely 

 a vestige of the pulp remains. This effect can be produced by im- 

 mersion in fresh water, by allowing the animal to remain in deterio- 

 rated water, or by the warming of the water by the manipulation used 

 in examination. In what way can this be produced but by the pres- 

 sure of the horny envelope on the pulp ? This, together with the 

 corrugation of the stalks of the cells in Campanularia volubilis, shows 

 a degree of irritability in the sheath entirely incompatible with an 

 inorganic and extravascular nature. The two parts are intimately 

 connected, governed in their growth by one principle, which not only 

 regulates their formation, but appears to be the source of the specific 

 differences. The polypes are but of secondary importance, and ex- 



P 



