SERTULAUIAD.E. 479 



ciliated on the surface, and distinctly irritable. There 

 are only two ova in each vesicle ; so that they do not 

 require any external capsules, like those of the Campamt- 

 laria, to allow them sufficient space to come to maturity. 

 On placing an entire vesicle, with its two ova, under the 

 microscope, we perceive through the transparent sides the 

 cilia vibrating on the surface of the coutained ova, and the 

 currents produced in the fluid within by their motion. 

 When we open the vesicles with two needles, in a drop of 

 sea- water, the ova glide to and fro through the water, at 

 first slowly, but afterwards more quickly, and their cilia 

 propel them with the same part always forward. They 

 are highly irritable, and frequently contract their bodies 

 so as to exhibit those singular changes of form spoken of 

 by Cavolini. These contractions are particularly observed 

 when they come in contact with a hair, a filament of 

 conferva, a grain of sand, or any minute object ; and they 

 are likewise frequent and remarkable at the time when 

 the ovum is busied in attaching its body permanently to 

 the surface of the glass. After fixing themselves, they 

 become flat and circular, and the more opaque parts of 

 the ova assume a radiated appearance ; so that they now 

 appear, even to the naked eye, like so many minute grey- 

 coloured stars, having the interstices between the rays 

 filled with a colourless transparent matter, which seems to 

 harden into horn. The grey matter swells in the centre, 

 where the rays meet, and rises perpendicularly upwards 

 surrounded by the transparent horny matter, so as to form 

 the trunk of the future zoophyte. The rays first formed 

 are obviously the fleshy central substance of the roots ; 

 and the portion of that substance which grows perpen- 

 dicularly upwards, forms the fleshy central part of the 

 stem. As early as I could observe the stem, it was open 

 at the top ; and when it bifurcated to form two branches, 

 both were open at their extremities ; but the fleshy central 

 matter had nowhere developed itself as yet into the form 

 of a polype. Polypes, therefore, are not the first formed of 

 this zoophyte, but appear long after the formation of the 

 root and stem, as the leaves and flowers of a plant." 



Attached tofucus and shells in abundance on the southern 

 coast of England, may be found the Plumularia cristata. It 



