Muscular Ttem.—The muscular system of Bowerbankia is described as follows.
For the process of retraction two distinct sets of muscles are provided ; the one acting upon the animal, the other upon the flexible part of the cell.
The muscles for the retraction of the ani mal are contained in the visceral cavity, and consist of two bundles of delicate thread-like chords (A. 56, 8 and 9) ; the one set, arising from the bottom of the cell, to be inserted about the base of the stomach ; the other, also arising from near the bottom of the cell, though generally at the opposite side from the former, and passing up free by the side of the pharynx, to be inserted around the line of junction of this organ with the base of the tentacula. The muscles provided for the retraction of the opercu lum, or flexible portion of the cell, have their origin from the inner surface and near the top of the stiff part, and are inserted into the flexible portion on which they act. They are most distinctly seen when the flexible operculum is completely drawn in; at which time the latter is folded up so as to occupy the axis of the upper part of the cell, and to it the muscles are seen extending from the oppo site sides of the cell from which they have their origin. They consist of six flattened bundles of fibres, having a triradiate arrange ment. The upper three sets (fig. 60, a, 3) act upon the upper part of the cell, and are in serted into it. The lower three (fig. 60, a, 4) are smaller, and are for the purpose of re tracting the bundle of setm with which it is crowned.
These fasciculi afforded Dr. Farre an ex cellent opportunity for investigating the struc ture of this form of muscle. It would appear as if muscular fibre were reduced to its sim plest condition. The filaments are totally disconnected, and are arranged the one above ' the other in a single series. They pass straight and parallel from their origin to their inser tion, and have a uniform diameter through their whole course, except that each filament generally presents a small knot upon its centre, which is most apparent when m 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 appear ance of cross-markings, nor of a linear arrange ment of globules. These muscles, though apparently attached to the inner walls of the cell, must yet have the membranous parietes of the body interposed between their inser tions and these walls. In the lower part the integument is only occasionally seen separate from the walls of the cell, but above it may be easily discerned in the expanded animal, passing up to be inserted around the tenta cular ring, and thus distinctly bounding this part of the body, which is always free within the expanded operculum.
The operation of this mechanism in retract ing the animal within its cell is as follows. The tentacula, from being expanded in the form of an inverted cone, are brought together into a straight line, and immediately begin to descend (fig. 60, d). Their descent is effected by the contraction of the muscle ( fig. 56. 9) which passes from the base of the cell to the tentacular ring, whilst at the same time the stomach is drawn down by its retractor (fig. 56. 8). The whole body, how
ever, does not descend in a mass, but must be folded up in a somewhat complicated manner, in order that the cell may completely enclose it. For this purpose the cesophagus sur mounted by the tentacula descends first, whilst the integument of the upper part of the body begins to be inverted at the point where it has its insertion around the tentacular ring. As the descent of the tentacula proceeds, the inversion of the integument continues forming a sheath around them (fzg. 60, c), until the extremities of the arms have descended to a level with the top of the unyielding portion of the cell. The animal is now drawn com pletely in, the stomach brought close to the bottorn of the cell, and the cesophagus bent in the form of the letter S ; the tentacula lying straight in the axis of the cell, enclosed in their tegumentary sheath, and so sepa rated from the fluid in the general visceral cavity, the centre of which they have the appearance of occupying, while in fact they are external to it. The animal being thus re tracted, the next step of the process is to draw in the upper part of the cell after it. This process, however, always commences before the retraction of the body is com pleted, and by the time that the ends of the arms are on a level with the base of the setw, the latter are brought together in a bundle, and begin to descend apparently by the action of the lower of the two sets of opercular retractors above described. Their descent, like that of the tentacles, takes place exactly in the axis of the upper part of the cell, and is accompanied by an inversion around them of its flexible portion, similar to that of the integument of the body around the tenta cula dunng their descent (fig. 60, b). Whilst the lower set of muscles are drawing down the setce, the upper set complete the retraction of the flexible part, and the whole operculum is thus packed closely in the upper part of the cell, the end of which now presents a triangular indentation, corresponding with the triangular arranonment of the opercular re tractors (fig. 606, a). Thus the whole pro cess of retraction may be easily accounted for, and the office of each set of muscles satisfactorily explained ; but the protrusion of the animal is effected by a totally different mechanism, viz., by the action of a set of transverse muscles acting upon the lining membrane of the cell, so as by their contrac tion to diminish considerably the dianzeter of the visceral cavity, and consequently exercise a pressure upon the fluid which it contains. The effect of this will be to elongate the body in the direction in which it is most free to move ; but Dr. Farre supposes that the act of protrusion is materially assisted by the co operation of the alimentary canal, which un doubtedly has the power of straightening itself from the sigmoid flexure into which it is thrown when the animal is retracted ; and that this is the case appears the more pro bable, when we reflect that in the case of the simple hydriform polypes the advance and recedinr, of the animal in its cell is entirely effectecrby the action of the parietes of the body, which are analogous to the alimentary canal in the present case, the hydriform po lypes possessing no distinct muscles to assist in these operations.