This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1203). The food appears to be retained for a considerable time in the stomach, and may be frequently seen to be regurgitated into the gizzard, whence, after having been again submitted to its operations, it is returned to the stomach. Here it is rolled about by the contraction 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 occasionally seen projecting through the pylorus into the intestine, and rotating rapidly in the direction of the axis of the orifice. In an animal having a similar form of pylorus to this, but in which the parts were more transparent, the cilia, by which this rotation is effected, were distinctly perceptible, surrounding the orifice.
(1204). The granular matter, after rotating for some time at the pylorus (a provision for preventing its too rapid escape from the stomach), passes into the intestine, where it accumulates in little pellets, that are rapidly pushed, by the contraction of the intestine, towards the anal orifice, through which they are expelled from the body.
(1205). The tube or cell inhabited by this Polyzoon is of exquisite structure, and the mechanism concerned in the protrusion and retraction of the animal of great simplicity and beauty.
(1206). The inferior two-thirds of the cell in the species under consideration is hard and corneous, but perfectly transparent: the upper third, on the contrary, is flexible, and so constructed as to form a very complete operculum whereby the entrance is guarded. The flexible part consists of two portions, the lower half being a simple continuation of the rest of the cell, while the upper is composed of a circle of delicate bristle-shaped processes or setae, which are arranged parallel to each other around the mouth of the cell, and are prevented from separating beyond a certain distance by a membrane of excessive tenuity that connects them; this membrane is evidently analogous to the infundibular termination of the cells of polyps already described.
(1207). When the Polyzoon retires into its abode, the setae and soft termination of the cell are gradually folded inwards, in the manner exhibited in the annexed figures (fig. 239) representing the various stages of the process. The oesophagus, surmounted by its 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 base of the tentacles (c.) As the descent of the tentacnla proceeds, the inversion of this membrane continues; and when the extremities of the arms have reached the level of the extremities of the setae, it is seen to form a complete sheath around them. The animal being thus retracted, the next part of the process is to draw-in the upper portion of the cell after it. The setae are now brought together in a bundle (fig. 239,2, a) and are gradually drawn inwards, inverting around them the rest of the flexible portion of the cell, until they form a close fasciculus (fig. 239, 3 & 4, a) occupying the axis of the opening of the tube, constituting a complete protection against intrusion from without.
Fig. 239. Bowerbankia, showing the opercular apparatus.
(1208). The muscular system exhibits the earliest appearance of muscular fibre. The filaments are unconnected by cellular tissue, and have a watery transparency and smooth surface; neither do they exhibit cross markings, or a linear arrangement of globules, even when examined under the highest powers of the microscope.
(1209). The muscles may be divided into two sets: - one for the retraction of the alimentary apparatus; the other acting upon the setae around the mouth of the cell, and serving for the inversion of its flexible portion. The bundles of muscular fibre which act upon the alimentary canal are two in number, and arise from near the bottom of the cell: one of these is inserted into the stomach (fig. 238, a, 8); the other passes upwards along the side of the oesophagus (fig. 238, a, 9), to be attached in the vicinity of the tentacula: the latter fasciculus is evidently the great agent in drawing the animal into its retreat, and in doing so it throws the alimentary canal into close sigmoid folds.
(1210). The muscles that close the operculum are arranged in six distinct fasciculi; they arise from the inner surface of the upper hard part of the cell, and act upon the upper flexible portion of the tube and upon the setae (fig. 239, d d).
(1211). The mode in which the protrusion of the tentacula is effected is not so easily explained; it would seem that the lining membrane of the shell is furnished with circular muscular fibres, so disposed as by their action to compress the fluid contained in the visceral cavity, and thus tend to elongate the body. Dr. Farre, however, believes the alimentary canal itself to be the great agent in effecting this object; and he conceives it to possess a power of straightening itself from the flexures into which it is thrown during the retracted state of the animal.
(1212). The Flustrae and Eschaeje are intimately allied to Bower-banhia in all the details of their structure, as we are assured by the researches of Dr. Milne-Edwards concerning these singularly aggregated forms of marine Polyzoa*.
(1213). The cells of the Flustrae and Escharce are disposed side by side upon the same plane, so as to form a common skeleton of a coriaceous or horny texture. The individual cells, which are extremely minute, vary in shape in different species; and the orifice of each is generally defended by projecting spines, or sometimes by a moveable operculum, or lid, that closes the orifice in the contracted state of the animal. The extension of one of these skeletons is effected by the regular addition of new cells around the circumference of the Flustra, those of the margin being, of course, the most recent; and the latter are not unfrequently found inhabited by healthy animals, whilst in the older or central ones the original occupants have perished.