The digestive apparatus is no longer a closed sacculus, as in the Polyps, but, on the contrary, presents a very elaborate structure, consisting of a gizzard, stomach, and distinct intestinal canal, terminated by an anal aperture, though which the faces are ejected; and, more over, the whole of the digestive viscera float loosely in a visceral cavity wherein they are suspended. In addition to this, the mouth of the cell occupied by the Bryozoon is defended by a most delicate and complicated opercular apparatus, requiring a very perfect set of mus cles to perform all the required movements connected with the protrusion and retraction of the body, so that there is abundant reason for separating the animals in question from the Acrite Zoophytes.
As relates to their muscular system, rudi mentary in its development as it must still ne cessarily be, many circumstances of great inte rest have been brought to light by the patient investigations of Dr. Arthur Farre, to whose valuable memoir* we are indebted for the fol lowing particulars, which we give at some length, more especially as they will serve not only to elucidate this part of our subject, but to correct several important errors that have been promulgated relative to the ROMERA, an important class of animals next to be noticed, of very analogous structure.
For the purpose of drawing the protruded Bryozoon into its retreat two distinct sets of muscles are provided. one set acting upon the animal and the other upon the flexible opercu lum that closes the cell. The muscles for the retraction of the animal are contained in the visceral cavity, and consist of two bundles of delicate thread-like cords, the one set arising from the bottom of the cell to be Inserted about the base of the stomach, the other also arising from the bottom of the cell and passing up free by the sides of the pharynx to be inserted around the line of junction between this organ and the tentacula.
The muscles provided for the retraction of the operculum consist of six flattened bundles of fibres, which act upon the flexible portion of the cell and a delicate circle of seta placed around its orifice. It is at once evident there fore that the muscular system in the Bryozoa is capable of great precision of action, and the fascicul i are most accurately adjusted. Neverthe less if the intimate structure of this form of muscle be investigated, it is found not to have attained to full perfection. " It would appear," says Dr. Arthur Farre, " as if muscular fibre were here reduced to its simplest condition. The filaments are totally disconnected, and are arranged one above the other in a single series. They pass straight and parallel from their ori gin to their insertion, and have a uniform dia meter throughout their course, except that each filament generally presents a small knot upon its centre, which is most apparent when it is in a state of contraction, at which time the whole filament also is obviously thicker than when relaxed. The filaments have a watery transparency and smooth surface, and under the highest powers of the microscope present neither an appearance of cross markings nor of a linear arrangement of globules." Besides
the retractor muscles in the Bryozoa there like wise exists a muscular membrane which lines the cell, and forms the parietes of the body, in which fibres are distinctly apparent, running transversely : these by their contraction com pressing the visceral cavity and the fluid which it contains will tend to elongate the body of the Bryozoon, and assist in effecting its protru sion ; although, as Dr. Farce supposes, this process is principally accomplished by the cooperation of the alimentary canal, which has the power of straightening itself from the sig moid flexure, into which it is thrown when the animal is retracted.
The condition of the nervous system in the Bryozoa has not been as yet made out, a cir cumstance at which no one will be surprised who considers the extreme difficulty of micro scopic researches concerning the structure of animals so minute as these ; but from the close affinity that there decidedly is between these animals and the Rotifera, there can be little doubt that a similar arrangement exists in both.
In the ROTIFERA or Wheel Animalcules, the nervous system, according to Ehrenberg, con sists of several filaments communicating with minute ganglia dispersed in different parts of the body, although without any obvious arrange ment or symmetrical disposition, so that the muscular apparatus in these beautiful animals presents itself in a very perfect state of dere lopement. The ciliated organs around the mouth, which are apparently the representatives of the ciliated arms in the Bryozoa, are retracted by a special set of muscles derived from the in terior of the membrane that lines the shell and circumscribes the visceral car ity,and the antago nists to these are the delicate parietes of the visceral cavity itself, which acting upon the fluid therein contained, causes the extrusion of the ciliated lobes, whenever the wheel-like organs are required to be put in motion.* But besides the muscular bands, that, in the Ruffen', are appropriated to the protrusion and retraction of the wheel-bearing organs, others are connected with a peculiar prehensile apparatus placed at the hinder extremity of the body, and forming an instrument of very great importance in the economy of these creatures. It consists of a prehensile forceps, the blades of which are worked by distinct muscles ; and by the assistance of this organ the action of the wheel-like cilia is at once changed from that of a locomotive power into a means of procuring and seizing food. If these forceps are not employed, the apparently rotatory organs pro pel the little animal rapidly through the water ; but does it choose to take hold of some foreign body by means of its forceps, and thus anchor itself in a given spot, the use of the wheels is entirely changed, their rotation merely pro ducing cun'ents or rather a powerful whirlpool in the water, which sucks from a distance every thing within its influence, and thus brings food into the mouth.