In some species of Bryozoa there are only two sets of opercular muscles, whilst in others one set only is perceptible.
Alimentary systenz. — In Bowerbankia the whole alimentary apparatus has been minutely described and figured by Dr. Farre.# The ten tacula are united together at their base to form a circle, in the centre of which is the mouth, and from which descends tbe ceso phagus, bulging a little at its commencement, and then contracting and passing down nearly straight to its termination. The parietes of the cesophagus, especially at the upper part, which may be more correctly denominated the pharynx (fig. 56, a, 1), are thickly studded with minute oval spots, arranged closely in contact with each other. The whole organ appears to be highly irritable, and contracts vigorously when food is intro duced into it.
At the termination of the cesophagus is a small distinct cardiac orifice (fig. 56, a, 2) opening into a small globular cavity (3), of singular construction, that appears to perform the office of a gizzard, the parietes of which are thicker than any other part of the ali mentary canal. This gizzard contains two dark round bodies, placed opposite to each other, from each of which dark lines are seen radiating. In the space between these two dark bodies may be seen a number of squami form spots, arranged closely in contact, and presenting a beautifully regular tesselated appearance, which, on minute examination, is found to consist of a pavement of gastric teeth.
The gizzard opens downwards into the true digestive stomach (4), an oblong cavity terminating below in a blunt extremity. The entire walls of the stomach are thickly studded with spots of a rich brown colour. These appear to be hepatic follicles, and to prepare a fluid that tinges the whole organ, as well as its contents, of a rich brown hue.
From the upper part of the stomach, and by the side of the entrance from the gizzard, arises the intestine (6) by a distinct pyloric orifice (5) that is surrounded by vibrating cilia. The intestine is narrow, and passes up straight by the side of the cesophagus, from which it is entirely separate and free, and terminates by a distinct anal orifice in the delicate parietes of the body, close to the outer side of the tentacular ring. The pari
etes of the intestine are marked with pale spots, and, like those of the whole of the alimentary canal, possess a high retractile power. The animal, when in full vigour, is seen projecting from its cell with the arms extended and the cilia in active operation, the upper part of the _body being frequently turned from side to side over the edge of the cell, the extremity of which, from its peculiar flexibility, moves with it. The particles carried to the mouth by the action of the cilia, after remaining a little while in the pharynx, are swallowed by a vigorous contraction of its parietes, and carried rapidly down the cesophagus and through the cardia into the gizzard, which expands to receive them. Here they are submitted to a kind of crushing process, the parietes of the organ contracting firmly upon them, and the two dark bodies being brought into apposition. Their residence, however, in this cavity is only momentary, and they are immediately propelled into the true stomach below, where they become mixed with its contents, which, during digestion, are always of a rich brown colour, being tinged with the secretion of its parietal follicles.
The food appears to be retained for a con siderable time in the stomach, and may be seen to be frequently regurgitated into the gizzard, whence, after having been again sub mitted to its operations, it is returned to the stomach. Here it is rolled about by the con tractions of its parietes, and at its upper part is frequently submitted to a rotating motion. This rotation of particles is chiefly near the pyloric orifice, and a mass may be often seen projecting through the pylorus into the intes tine, and rotating rapidly in the direction of the axis of the orifice. This rotation is ef fected by the action of cilia surrounding the pyloric orifice, which, in very transparent specimens, are distinctly visible with high powers of the microscope.