SERTULARIADiE : PLUMULARIA. 91 



a fine figure of the coralline in the centre of the curious frontispiece 

 to Ellis's Essay ; and the magnified figure in tab. 38 is a more cor- 

 rect representation of the cells than that given in tab. 7, which has 

 been drawn from a dried specimen. The ovarian vesicles are of 

 uncertain occurrence, and I have seldom seen them : they are scat- 

 tered irregularly on the branches, stalked, ovate or pear-shaped, with 

 a short tubulous aperture, and occasionally wrinkled longitudinally 

 when dry. 



" This species is very common in the deeper parts of the Frith 

 of Forth : its vesicles are very numerous, and its ova are in full 

 maturity at the beginning of May. The ova are large, of a light- 

 brown colour, semi-opaque, nearly spherical, composed of minute 

 transparent granules, 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 Campanularia, 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 ciliae vibrating on the surface of the contained 

 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 after- 

 wards more quickly, and their cili« 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 they have fixed, 

 they become flat and circular, and the more opake 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 inter- 

 stices 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, sur- 

 rounded 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 sub- 

 stance which grows perpendicularly upwards, forms the fleshy cen- 

 tral 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, 



