112 Dr Grant (yii the Structure and Nature of Flustra. 



zoophytes. Its branches are broad, thin, semitransparent, studded with 

 small reddish-brown spots, generally dichotomous, often trichotomous ; the 

 trunks of the branches have thick, yellow, opaque margins, and their free 

 extremities are very thin, membranous, transparent, and rounded or lobed. 

 In the dried state the branches have a glistening membranous surface, they 

 produce distinct effervescence, and coil up when touched with nitric acid, 

 indicating the presence of carbonate of lime in the homy texture of their 

 cells. They are so delicate that we rarely find a specimen in which the 

 branches are not broken at their extremities, or perforated with ragged 

 holes, and they are very often studded on both sides with small patches of 

 the Flustra dentata, in the same manner as the Flmtra foliaca is very much 

 infested in the Frith of Forth with creeping branches of the Cellaria reptans. 

 There are no tubular roots in this species as there are in the F. tintncata ; the 

 compact base is formed of condensed cells, which originally contained polypi. 

 The polypi are deficient near the base, as in other flexible branched zoo- 

 phytes, from the constant bending and pressure at that part, which gradually 

 extend and approximate the sides of the cells, and thus render the stems 

 more compact, flexible, and strong, to sustain the increasing weight of the 

 branches, and consequent increased influence of the waves. This takes 

 place likewise in the stems of branched zoophytes without polypi, and may 

 be compared to the condensation of cellular substance into membrane and li- 

 gament in higher animals. It is by rearing the ova of this species on the 

 surface of watch-glasses, that I have found its first formed parts to consist 

 of polypiferous cells, and not of tubular roots, as in many other zoophytes, 

 although the same may be ascertained by a careful examination of these hard 

 parts. The cells are arranged with remarkable exactness, in perpendicular 

 straight lines, and in curved rows diverging on each side to the margin. It 

 appears much more important in the economy of a flustra to preserve this 

 exact arrangement, than to perfect the forms of the individual cells, as we 

 often observe the cells at the commencement of the new rows assume a small 

 and distorted form, in order to adjust them to the precise line of arrange- 

 ment of the neighbouring cells. The cells are all nearly of the same size, 

 in whatever part of the branches we observe them, and whether on young or 

 old, large or small specimens. The cells are about a third of a line in length, 

 and half as much in breadth. They are widest in the middle, slightly ta- 

 pering and arched at the summit, and contracted to about a third of their 

 breadth at the base- They open by an arched and folding aperture near their 

 wider extremity, and all the apertures are placed on the same side of the 

 branch, which is probably the most pendent in the natural state. As the 

 cells have only one aperture, and are arranged in a single plane, we find one 

 side of the branches in this species entirely free from apertures; thisNshut 

 side of the branches is the most frequent seat of the Flustra dentata, but the 

 side containing the apertures is likewise often attacked by this parasitic spe- 

 cies. The anterior part of the cell consists of a thin transparent membrane. 

 The margins of this membrane are supported and protected by several fasci- 

 culi of straight slender calcareous spicula, which are attached to the solid 

 sides of the cell, and extend inwards along the surface of the membrane. 

 These spicula are all of the same size and form ; they are less than the tenth 



