410 DR. A. FARRE ON THE STRUCTURE OF THE CILIOBRACHIATE POLYPI. 
centre. As they grow they thrust aside the surrounding cells, and the number of their 
sides increases until they acquire the irregular hexagonal form of the adult. In the 
oblique position of the cells (in which they look like a new growth encrusting the 
old mass like a Flustra ) the young cells are less angular, and arranged more regu- 
larly at the spreading edge (fig. 5.). 
This species afforded an opportunity of examining also the reproductive gemmules. 
These are readily seen in spring as minute whitish points just below the surface of 
the mass (fig. 3. a a.). Sometimes they are of a darker colour, and exceedingly nu- 
merous, appearing to occupy almost the whole substance. If one of these points he 
carefully turned out with a needle it is found to consist of a transparent sac (fig. 20. a.), 
in which are contained generally from four to six of the gemmules, which, as soon as 
the sac is torn, escape and swim about with the greatest activity, affording a most 
interesting subject for microscopic investigation. 
When viewed with a power of forty, linear measure, they are seen to be of an oval 
or rounded form (fig. 20. b and c.), convex above, and nearly plane below, and fringed 
at the margin with a single row of cilia, which appear to vibrate in succession round 
the whole circumference. Under an amplification of 120 they assume a different 
aspect (fig. 21. and 22.), and their minute structure is clearly discerned. Viewed as 
opake objects, both the body and cilia have a silvery whiteness, but by transmitted 
light the former appears of a dark brown, and the cilia of a golden yellow colour. 
Upon the most convex part of the body, which is not generally in the centre, but 
leaning to one side, are set from three to five prominent transparent bosses surrounded 
by a circle ; and other circles are seen extending to the base of the body, the extreme 
margin of which is bounded by a row of prominent tubercles. These marginal tubercles 
are from thirty to forty in number, and from the circumstance of the cilia arising from 
them, it is probable that they are for the purpose of governing their motions, and 
therefore analogous to the muscular lobes of Hydatlna senta. No structure, how- 
ever, could be detected in these, nor in any other part of the body beyond a mere 
granular parenchyma. 
Fig. 22. Under this power the whole character of the ciliary motion is changed, 
and it is seen that what before appeared to be a single cilium is in fact a wave of 
cilia, and that their motion, instead of being in the direction of the circumference of 
the disc, is at right angles to this. The ciliary phenomena are the most readily ob- 
served when the gemmule is nearly at rest, or has become languid ; it then lies either 
with the convex or the plane side uppermost, and with the cilia, which are of great 
length, doubled in the middle upon themselves (fig. 21.), so that their extremities are 
brought back nearly to touch the margin of the disc from which they arise. The 
whole fringe of cilia is then suddenly unfolded, and after waving up and down with 
a fanning motion they are either again folded up towards the under surface of the 
body, or they commence their peculiar action. As the cilia have the appearance of 
moving in waves round the disc (fig. 22.), each wave may be thus analysed. From a 
