Dr Grant on the Structure and Nature ofFlustra. 117 



ova have escaped, we frequently observe a few remains of the former polypus 

 lying at the place where the body of the polypus bifurcated, and where the 

 principal connection seems to exist between the polypus and the axis ; we 

 likewise perceive numerous monades and other animalcules busily employed 

 in consuming the remains of the dead polypus. The ovum, even before ar- 

 riving at maturity, exhibits very obvious signs of irritability, frequently con- 

 tracting different parts of its surface, and shrinking backwards in its cell ; the 

 ciliae on its surface are likewise observed in rapid motion within the cell, as 

 in the ciliated ova of other zoophytes. The mature ova are often found with 

 their small end projecting from the opening of the cells, and their final escape 

 is aided by the incessant vibrations of the ciliae covering their surface, by the 

 ova contracting themselves in their lateral direction, by the waves agitating 

 the branches of the flustra, and by the same incomprehensible laws which re- 

 gulate the formation and growth of the ova, and the whole economy of this 

 zoophyte. 



When the ova of the F. carbasea have escaped from the cells, and are ob- 

 served swimming to and fro in a watch-glass with sea-water under the micro- 

 scope, we perceive that the small end which first escaped from the cell is car- 

 ried foremost, and the broad posterior end has now expanded into a broad cir- 

 cular zone, giving a flatness to that extremity. The cilise are longest in the 

 centre of the broad extremity as in other ova *, and become gradually smaller 

 towards the narrow end. When torn and examined on a plate of glass under 

 the microscope, the whole ovum appears composed of very minute gelatinous 

 granules or monade-like bodies, without any external capsule or internal cal- 

 careous matter. They are very irritable, and are frequently observed to con- 

 tract the circular margin of their broad extremity, and to stop suddenly in 

 their course when swimming ; they swim with a gentle gliding motion, often 

 appear stationary, revolving rapidly round their long axis, with their broad 

 end uppermost ; and they bound straight forward, or in circles, without any 

 other apparent object, than to keep themselves afloat till they find themselves 

 in a favourable situation for fixing and assuming the perfect state. The 

 time of their remaining in this free and moving state varies from a few hours 

 to about three days, according to circumstances. When placed in a watch- 

 glass, immersed in a vessel of pure sea-water recent from the sea, and kept in 

 the cavity of the glass, by a careful management, they generally fix within the 

 space of six hours from the time of their escape from the cells. The slight- 

 est agitation when they are about to fix, causes them to recommence, and con- 

 tinue for some time, their gliding motions ; and if again separated from the 

 surface of the glass when they have begun to fix, they generally remain free, 

 and perish. During the process of fixing, they exhibit no peculiar appearance 

 or change of form"; they appear simply to lie on their side, and the cilise con- 

 tinue to vibrate over the whole surface, producing a constant current in the 

 water, and clearing the space immediately surrounding the ovum ; on agita- 

 ting gently the water, however, we now find that it can no longer move from 

 its place. I have found the ova of the F. carbasea remain three days in this 

 fixed recumbent position without undergoing any perceptible change of form, 



* See Edinburgh New Philosophical Journal, December 1826, p. 129. 



