In order to understand how growth is ac complished in these tube-clad zoophytes, it will b2. necessary to refer once more to the precel ing diagram (fig.55). The tegumentary layer of the zoophyte (fig. 55, b) is at first quite soft and expansible, the hard corneous matter by which it is consolidated being afterwards superadded to its texture. Whilst growth is in progress, therefore, this outer layer shoots upwards in conformity with the pattern to which it belongs ; but whilst the top of the tube retains its softness and power of growth it is continually fortified below by the depo sition of the horny matter which gives it solidity : growth can therefore only proceed at the extremity of every branch where this layer remains capable of further development ; for no sooner is it solidified than it remains permanently unchangeable. Hence it is that these zoophytes differ so remarkably frora plants in the character of their arborescence : in the latter the stem is increased by constant additions to its thickness, but in the case of the Sertularia no such thickening is possible ; so that both stem and branches retain the same diameter throughout, however much their rainifications may be extended. As the growth of the tegumentary layer thus proceeds in one direction only, except when the development of polype-cells calls for its lateral expansion, the nutritive layer within continues to grow pari passu, and from it the polypes are pro duced as the .cells become ready to receive them.
BaYozoA (Ehrenberg), CILIOBRACIIIATE POLYPI (Farre).— The Bryozoa, although closely resembling some of the simpler Poly pifera, described in the preceding pages, with which, indeed, until a very recent period, they were confounded by zoological writers, differ from them in so many essential points of their structure, that, but for the convenience of description, we should have preferred to re gard them as a distinct class, exhibiting a much higher phase of organisation than any of the nudibrachiate races. In all the fami lies of Polypifera we have as yet had occasion to examine, it will have been noticed that the tentacular apparatus around the mouth, al though very generally pinnated, are quite devoid of cilia ; but in the Bryozoa one of the most obvious circumstances observ able in their organisation is, that all the circumoral arms are crowded with vibratile organs, the play of which, when in action, is exceedingly energetic, producing rapid cur rents in the surrounding water, and thus hurrying towards the mouth of the animal whatever substances may come into the neigh bourhood of the vortex so produced, and in this way securing an abundant supply of food, almost without exertion on the part of the creature itself. From this most conspicuous character, common to the entire group, Dr. Arthur Farre was induced to propose for them the name of Ciliobrachiata. It is in
their internal economy, however, that their chief points of distinction are to be sought. Like the ordinary polypes, most of these little animals inhabit cells of different shapes and various degrees of density. These cells are sometimes calcareous and opaque, but in very many genera so thin and diaphanous that nothing is more easy than to examine, by means of the microscope, the anatomy of the animal within. When thus examined, the differences between a Bryozoon and an ordi dinary polype become immediately manifest, and may be briefly stated as follows.
In the nudibrachiate polypes the stomach is a simple sacculus unprovided with any intestinal tube or anal orifice, so that after taking food the egesta are necessarily ex pelled through the oral opening ; but in the Ciliobrachiata, not only is the stomach found to be floating loosely in a visceral cavity, and of very complete structure when compared with the digestive sacculus common to the pre ceding tribes, but it terminates in a complete intestinal canal, provided with a distinct anal orifice, through which the fwces are discharged. Accompanying this advanced condition of the alimentary apparatus all the other systems assume a more elevated type of structure, as will be immediately apparent from the details of their anatomy, upon the consideration of which we are about to enter. Much, doubt less, yet remains to be made out in the eco nomy of these animals ; still the researches of Ehrenberg*, Milne Edwards, Audouint, Thompson I, Farre §, and Van Beneden II, have already put us in possession of most important information concerning them, which promises to open a yet wider field for discovery.
The cell of Bowerbankia (fig. 56), as de scribed by Dr. Arthur Farre, is cylindrical, and closely embraces the body of the animal ; it is of a firm unyielding consistence in the lower two-thirds of its extent, but terminates above by a flexible portion, which serves to protect the upper part of the body when the whole is expanded, in which state it is of the same diameter as the rest of the cell ; but when the animal retracts, this portion is folded up and drawn in after it, so as to close its mouth. The flexible part consists of two portions, the lower half being a simple continuation of the rest of the cell, the upper consisting of a row of delicate bristle-shaped processes, or setx, which are arranged parallel with each other round the top of the cell, and are prevented from separating beyond a certain distance by a membrane of excessive tenuity which sur rounds and connects the whole. This arrange ment is common to all the species possessing a cylindrical cell ; but the length of' the setm is very variable; indeed they are sometimes so stunted in their development that their pre sence is hardly recognisable.