On the Anatomy of Forbesia
II.—On the Anatomy of Forbesia. By H. D. S. Goodsir, M.W.S., Acting Assistant-Surgeon of H.M.S. Erebus.—(Transmitted by the Author from Disco Island, Baffins Bay, in June 1845.)
The polype-like animal for the reception of which this genus has been proposed1 is remarkable, both on account of its gigantic size and its peculiar structure. It has the general appearance of a vorticella enormously magnified, with the addition of distinctly developed tentacula, in which particular, and in other respects, it has a great resemblance to the pedicellina of Sars. It differs from vorticella and pedicellina, however, in the want of an alimentary tube and anus; having only an oral aperture, like a hydroid polyp, to which it would appear to be nearly allied.
The individuals examined were from five to six inches in length, including the head, which was about an inch. The head was half, the peduncle one-eight of an inch in diameter2. (Fig. 1.) The oral surface of the head is truncated, concave towards the centre, where the mouth appears as a linear slit. The lips are thick, rounded, and fleshy, with ridges and intervening furrows at regular intervals. A double row of long fleshy papillae also surrounds the mouth, and the integument, where it covers the lips, presents the aspect of a mucous membrane; but over the rest of its extent it has a very different appearance, being reticulated like the surface of a delicate flustra. Several rows of fleshy papillae surround the head, somewhat nearer its oral than its peduncular extremity. These papillae can be distinctly seen only on one side of the head, being apparently obsolete on the other. Numerous clavate horny spines are attached to the lower half of the peduncle; and a few of them may be seen nearer the head.
Integument.—The integumentary membrane is an extremely beautiful object, consisting of a thing reticular structure of considerable strength, with its external surface of a metallic lustre. Magnified, it much resembles muslin; it fibres crossing one another at right angles in a most regular manner. Numerous lines appear on it at equal distances, the membrane looking as if thrown into folds; or its fibres to be more thickly interwoven, as in a seam. This would appear to be an arrangement for admitting of the extension of the membrane when the head is distended by food or water, for these folds do not exist in the integument of the peduncle. A number of small, clear, circular discs are also visible on the integumentary covering of the head. In the centres of these discs are minute dark coloured spots, which resembled apertures. The papillae which encircle the mouth, and the similar processes which surround the body, have a general resemblance to integumentary appendages; but, as will be shown in the sequel, must probably be referred to other systems, being merely covered by the skin.
The horny spines already alluded to are, however, developments from the integument. They are scattered irregularly over the basal extremity of the peduncle, and appear to be merely organs of defence. Their length somewhat exceeds one-eight of an inch. They are club-shaped, or conical with their apices attached, and an open conical pit hollowed out in their free extremities; and when magnified, they exhibit an external layer of longitudinally striated texture; then a concentrically laminated structure; and internally, a light-coloured substance, like the pith of a quill, slightly marked across its axis, and bulging considerably about its middle. Th external and intermediate layers have a horny consistence and appearance; the former apparently cuticular. The external laminæ of the latter are the largest, and therefore, probably, the first formed from the surface of the enclosed conical pulp. These spines are firmly connected to the skin, and are extracted with difficulty, but do not appear to have any muscular fasciculi attached to their basal extremities.
Internal Membrane.—A very delicate, opaque, flocculent membrane lines the internal surface of the integumentary covering of the head and peduncle, floating loosely within it, and enveloping the deeper structures. It originates from around the oral extremity of the gastric bulb, from between which and the skin it extends downwards into the peduncle. As already stated, this membrane floats free within the integument. This, however, is probably the result of incipient putrefaction or mechanical injury; and subsequent examination will, I think, prove that its external surface is loosely attached, by delicate connection texture. To the internal surface of the skin. Its internal surface is free, and forms the outer wall of a space which exists between it and the outside of the gastric bulb, with the membrane covering which, the internal membrane now under consideration appears to be continuous. This space or cavity communicates with the exterior by an orifice, to be afterwards described, in the fundus of the stomach, and appears to be of the same nature as the extended cavity which exists around the stomach, and in the axis of the helianthoid and asteroid polyps. The contents of this cavity consist of irregular masses of soft granular matter resembling mucus.
When this cavity is laid open, and the gastric bulb turned out, which is easily effected, as it is only suspended by its oral extremity, a small tubercle is observed at the fold formed by the reflection of the internal membrane upon the outside of the gastric bulb. This tubercle is situated immediately within and beneath the lips. A central depression but no distinct aperture could be detected in it.
Organs of Digestion.—The alimentary apparatus in this animal, as in the polyps, consists of a gastric cavity. When the internal membrane already described is divided, a bulbous mass, which contains the gastric cavity, is seen lying loose within it, suspended freely by its oral extremity. Viewed in its natural position, it has a considerable resemblance to, and is about the size of a date stone. The pyloric orifice is very perceptible at its free extremity, as large as the head of a pin, circular, with numerous radiating folds. One of the blades of a pair of scissors having been introduced through the mouth into the gastric cavity, then through the pylorus, so as to lay the cavity open, it may be observed that the external integument passing into and lining the cavity, leaves it through the pyloric aperture, and proceeding in the direction of the mouth, closely adheres to and covers the external surface of the gastric bulb. It appears to be again reflected, as has already been stated, at the junction of the gastric apparatus with the integument, passing on to be continuous with the internal membrane, described above. The gastric cavity has four walls, with an internal membrane, white, thick, and spongy, assuming a coarsely fibrous texture and a metallic lustre, often passing beyond the pyloric orifice upon the outside of the gastric bulb. Two dark brown, hard, horny teeth, of a compressed, conical, recurved form, are situated, one in each of two diagonal corners of the quadrilateral gastric cavity, a little way within the internal edge of the mouth. The papillae which encircle the middle region of the head possess a structure which inclines me to believe that they constitute a hepatic apparatus, although I have failed in detecting any communication between them and the gastric cavity, or the space in which the gastric bulb is suspended. Immediately under the integumentary covering of these papillae is a delicate membrane, in which are situated numerous elliptical spots, with their extremities produced and pointed. At the free extremity of the papilla, in or within the delicate membrane, is a large clear cell or nucleus; beyond which, in the body of the papilla, are numerous smaller cells, in and around which is an oleaginous fluid resembling bile.
If the cavities of these of papillae communicate with the space in which the gastric apparatus is suspended, although differing in form, their morphological position and connexion would indicate a similarity to the glandular fringes on the free margins of the meso-gastric folds of the asteroid polyps.
Organs of Reproduction.—The papillae which surround the mouth appear to contain the ovaries, or to constitute ovarian appendages, as in the tubularian polyps. Each papilla contains a mass of cells in various stages of development, the extremity presenting a clear spot similar to those in the presumed hepatic papillae.
On the external surfaces of the four sides of the gastric cavity, and covered by the membrane reflected upwards from the pyloric orifice, are four elongated masses, each divided longitudinally by a line or impression on its external surface. Each mass consists of a double series of transverse plates, connected in the middle by a longitudinal septum proceeding from the investing membrane, and the edge of which is indicated by the longitudinal line or impression already described on the external surface of the mass. The plates consist of numerous tubes, which contain minute cellules. The tubular masses are white, and do not present the structure or aspect of hepatic organs; and as they may be presumed to open into the cavity around the stomach, into which also it is probable, from their structure, the supposed ovarian papillae empty themselves, I am inclined to consider those four double masses as male organs. This hypothesis is supported by the arrangement of the reproductive organs in the asteroid and helianthoid polyps.
Explanation of Plate II
Fig. 1. Forbesia formosa of the natural size.
Fig. 2. The external integument laid open, to show the internal membrane, and the parts contained within it. a the external integument of the head laid over; b the external integument of the peduncle laid over; c the internal membrane passing down from off the gastric bulb and (testes?) into the peduncle; d the gastric bulb; e the oral aperture; f the pyloric orifice; g the inferior papillae; h the superior papillae.
Fig. 3. The internal membrane laid open, and the gastric bulb turned up to expose the space within the membrane in which it is suspended. a the gastric bulb; b the pyloric orifice; c the tubercle, with a central depression on one side of the oral fold of the internal membrane; d the external integument; e a mass of mucus contained in the cavity of the internal membrane; f the internal membrane.
Fig. 4. The microscopic structure of the membrane which invests the gastric bulb and (testes?), and is continued through the pyloric orifice from the lining membrane of the gastric cavity.
Fig. 5. The gastric bulb removed, and a portion of the investing membrane turned down to exhibit one of the four double packets of tubular laminæ, (testes?). a the oral aperture; b the oral papillae; c the investing membrane; d d one of the four double packets of tubular laminæ, divided longitudinally by a septum which passes in from the investing membrane; e the pyloric orifice; f a portion of the investing membrane turned down, showing the line of detachment of the longitudinal septum, and smaller transverse markings on the surface opposed to the (testes?).
Fig. 6. The gastric bulb laid open, to show the internal surface of it’s cavity. a the oral papillae; b the oral aperture laid open; c the external integument; d the internal surface of the cavity divided longitudinally by four corners into four surfaces, like the walls of a room; e e two conical compressed dark brown horny teeth, situated in two of the diagonally opposite corners of the cavity, about a third of its length from the oral aperture; f the pyloric orifice, with its radiating folds.
Fig. 7. A portion of the external integument magnified.
Fig. 8. Minute nucleated spots on the surface of the integument.
Fig. 9. The lining membrane of the gastric cavity magnified.
Fig. 10. A portion of the integument of the head magnified to show the folds, which are probably useful in allowing the distention of the head by food or water.
Fig. 11. One of the oral papillæ magnified to show its structure. a the free extremity of the papilla, within which is seen a clear space or nucleus, surrounded by b a cell-wall; c the integument of the papilla; d d ova (?) in various stages; e attached extremity of the papilla.
Fig. 12. One of the papillae which surround the head magnified. a its free extremity; b cellular structure, probably hepatic; c attached extremity; d delicate membrane with elliptical spots under the integument which covers the papilla; e peculiar bodies in the integument represented in
Fig. 13, and apparently of the same nature as those represented in Fig. 8.
Fig. 14. The extremity of a (hepatic?) papilla more highly magnified.
Fig. 15. The tubular structure of the (testes?) magnified.
Fig. 16. One of the clavate, horny spines attached to the external integument magnified. a its free extremity, which is hollow; b depth to which the cavity proceeds; c the attached extremity; d the integument.
Fig. 17. The pulp of the spine.
Fig. 18. A portion of the integument and internal membrane magnified; the spots on the former are here very distinct; the fibres of the latter are probably contractile or muscular.