At what time the nervous system is formed could not be detected.
When the cell has nearly reached its full development, the tentacular sheath is com pleted in the same proportion, the parietes of the cell become softened, and an opening is formed which brings the young polype into communication with the surrounding element. The Bryozoon has now attained its full deve lopment, and can expand its tentacula, but as yet there are no traces of the reproductive organs, which seem to he formed after all the others.
In Halodadylus reproduction by gemma tion is effected by the development of young animals and cells amongst the mature ones. The newly formed cells are triangular, and the animal looks like a mere spot in their centre. As they grow they thrust aside the surrounding cells, and the number of their sides increases until they acquire the regular hexagonal form of the adult.
The Halodactylus likewise afforded Dr. Farre an oliportunity of witnessing the second mode of reproduction common to the Bryozoa, namely, by the development of ciliated gem mules. These are readily seen in Spring as minute whitish points situated just below the surface of the mass (fig. 64, a). Sometimes The second cavity, which is the intestinal, has as yet no communication with the ex ternal world. As the formation of the ten tacula proceeds, the portion which is situated in front of them will become the sheath, and the other part the proper intestinal canal; the former cavity is, therefore, in all respects comparable to that which exists in the Tuni cata situated in front of the proper oral orifice and lined with the branchial vessels. The tentacles of these polypes, in fact, if connected by transverse canals and attached to the sheath, would transform the animals in this phasis of their growth into Ascidians.
As the tentacula are fortned by the pro longation of the tubercles which were their they are of a darker colour, and exceedingly numerous, appearing to _occupy almost its whole substance. If one of these points be carefully turned out with a needle, it is found to consist of a transparent sac, 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 ac tivity, affording a most interesting subject for microscopic investigation.
When viewed with a power of 40, linear measure, they are seen to be of an oval or rounded form (fig. 63, b), convex above and nearly, plane below, and fringed at the margin with a single row of cilia, which appear to vibrate in succession around the whole cir cumference.
Under an amplification of 120 they assume a different aspect (fig. 61), and their minute structure is clearly discerned. Viewed as opaque 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 transparent bosses, surrounded by a circle, and other circles are seen extending to the base of the body, which is bounded by a row of promi nent tubercles. These marginal tubercles are from thirty to forty in number; and from the circumstance of the cilia arising from them, Dr. Farre considers it probable that they are for the purpose of governing their motions, and therefore analogous to the muscular lobes of Hydatina senta and other Rotifera figured by Ehrenberg. No structure, however, could be detected in these, nor in any other part of the body, beyond a mere granular parenchyma. When thus highly magnified, it is seen that what examined with a lower power 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 it. The ciliary phenomena are the most readily observed when the gemmule is nearly at rest, or has become languid ; it then lies either with the convex or the concave side uppermost, and with the cilia, which are of great length, doubled in the middle upon themselves, 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.