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.
(1214). The facts observed by Milne-Edwards relative to the formation of these cells possess a high degree of interest, and materially support the views already given concerning the formation of the tubes of zoophytes in general, proving that the calcareous matter to which their hardness is owing is not a mere exudation from the surface of the animal, but is deposited in an organized tegumentary membrane, whence it can be removed with facility by means of extremely dilute muriatic acid. When so treated, a brisk effervescence is produced; the cells become flexible, and are easily separated from each other; but they are not altered in form, and evidently consist of a soft and thick membrane, fonning a sac containing the digestive organs of the creature. In this state the opening of the cell is no longer denned as it was before, but the membranous cell appears continuous with the tentacular sheath. We see, therefore, that in these creatures the cell is an integral part of the animal itself - not a mere calcareous crust moulded upon the surface of the body - being a portion of the tegumentary membrane, which, by the molecular deposit of earthy matter in its tissue, ossifies, like the cartilage of higher animals, without ceasing to be the seat of nutritive movement.
It is evident, likewise, that what is called the body of the Polyzoon constitutes, in fact, but a small portion of it, principally consisting of the digestive apparatus.
* "Recherches Anatomiques, Physiologiques, et Zoologiques sur les Eschares" (Ann. des Sci. Nat. for 1836).
(1215). As to the operculum destined to close the entrance of the tegumentary cell, it is merely a lip-like fold of the skin, the marginal portion of which acquires a horny consistence, while at the point where it is continuous with the general envelope it remains sufficiently soft and flexible to obey the action of the muscles inserted into it.
(1216). The tegumentary sac, deprived of its carbonate of lime, seems to be formed of a tomentose membrane, covered, especially upon its outer side, with a multitude of cylindrical filaments disposed perpendicularly to its surface and very closely crowded together. It is in the interstices left by these fibres that the calcareous matter appears to be deposited; for if a transverse section be examined with a microscope, the external wall is seen not to be made up of superposed layers, but of cylinders or irregular prisms arranged perpendicularly to the axis of the body.
(1217). But the above are not the only arguments adduced by Milne-Edwards in confirmation of this view of the mode in which these skeletons are held in vital connexion with the animal. On examining the cells at different ages, it is found that they undergo material changes of form.
(1218). This examination is easily made, since in many species the young spring from the sides of those first formed, and do not separate from their parents; each skeleton therefore presents a long series of generations linked to each other, and in each portion of the series the relative ages of the individuals composing it are indicated by the position which they occupy. It is sufficient therefore to compare the cells situated at the base, those of the middle portion, those of the young branches, and those placed at the very extremities of the latter. When examined in this manner, not only is it seen that the general configuration of the cells changes with age, but also that these changes are principally produced upon the external surface. For instance, in the young cells of Eschara cervicornis (the subject of these observations), the walls of which are of a stony hardness, the external surface is much inflated, so that the cells are very distinct, and the borders of their apertures prominent; but by the progress of age their appearance changes, their free surface rises, so as to extend beyond the level of the borders of the cell, and defaces the deep impressions which marked their respective limits.
It results that the cells cease to be distinct, and the skeleton presents the appearance of a stony mass, in which the apertures of the cells only are visible.
(1219). It appears evident therefore that there is vitality in the substance composing the stony walls; and the facts above narrated appear only explicable by supposing a movement of nutrition like that which is continually going on in bone.
(1220). The anatomy of these Polyzoa differs slightly from that of Bowerbankia. The crown of ciliated tentacula is inserted into the extremity of a kind of proboscis, which is itself enclosed in a cylindrical retractile sheath. From the margin of the opening of the cell arises a membrane equalling in length the contracted tentacles, and serving to enclose them when the animal retires into its abode. These appendages, thus retracted, are not bent upon themselves, but perfectly straight and united into a fasciculus, the length of which is nevertheless much shorter than that of the same organs when expanded.
(1221). By the opposite extremity to that fixed to the margin of the opening of the cell, the tentacular sheath unites with a tolerably capacious tube, the walls of which are exceedingly soft and delicate; and near the point of their union we may perceive a fasciculus of fibres running downwards to be inserted upon the lateral walls of the cell: these fibres appear to be striated transversely, and are evidently muscular; their use cannot be doubted. When the animal wishes to expand itself, the membranous sheath above alluded to becomes rolled outwards, everting itself like the finger of a glove as the tentacles advance. The muscular fasciculi are thus placed between the everted sheath and the alimentary canal, and by their contraction they must necessarily retract the whole within the cell.
(1222). The first portion of the alimentary tube is inflated, and much wider than the rest; it forms a kind of chamber, in which the water set in motion by the vibration of the cilia upon the tentacles appears to circulate freely. The walls of this chamber are extremely delicate: the soft membrane forming them is puckered, and appears traversed by many longitudinal canals united by minute transverse vessels; this appearance, however, may be deceptive.