142 Z. HYDROIDA. Plumularia. 



ii. 259. Templeton in Mag. Nat. Hist. ix. 466. Risso, L'Europ. merid. 



V. 31.3 I.a P. en faux, Blainv. Actinolog. 477. 



Hah. On shells and rocks near low water-mark and in deep water. 

 A common and very eleg-ant species, generally from 4 to 6 inches in 

 height, rising- in wide spiral turns, and sending out from its filiform 

 percurrent stem, at regulated intervals, alternate spreading plumous 

 branches which are placed one above the other on the outer side. 

 Pinnae alternate, bifarious. In young speciniens the branches are 

 two- ranked and alternate, and I have seen this character remain in one 

 specimen of considerable size. There are no cells on the spiral 

 stem, but they occur on the branches as well as on the pinnae, and 

 are arranged in two rows pointing alternately to opposite sides. There 

 is a fine figure of the coralline in the centre of the curious frontis- 

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

 correct 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 scattered 

 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 matu- 

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

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

 rent 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 suf- 

 ficient space to come to maturity. On placing an entire vesicle? 

 with its two ova, under the microscope, we perceive through the 

 transparent sides, the cilise 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 cilise 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 off 

 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 hkewise frequent and remarkable at 

 the time when the ovum is busied in attaching its body permanent- 

 ly 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 radiat- 

 ed appearance ; so that they now appear, even to the naked eye, like 



