Operculum

“ This is high, full of points like towers, and thick like a castle wall. When feeding, enjoying itself or moving around, its long neck and body are stretched out before it, armed with its hard operculum, which is like an iron shield, or the end of a battering ram. The operculum fits the entrance to its shell like a trap door. As soon as any danger is near it pulls in its head, and slams itself shut with a loud noise. ”

“ A genus of microscopic Foraminifera.
OPERCULUM. (A cover or lid.) The plate or plates with which many molluscous animals enclose the aperture of their shells, when retired within them. The operculum is sometimes horny, as in Trochus; testaceous or shelly, as in Turbo. It is spiral when from a central or sub-central nucleus, the successive layers take a revolving direction, as in Trochus. It is concentric or annular when the outside edge of each layer entirely surrounds the preceding one. It is unguiculated, when the laminæ are placed side by side, as in Purpura. ”

“ The cells in some species are provided with a pedunculate operculum, by which it is intended to designate a process, which arising by a short tube from the anterior wall of the cell, immediately beyond the inner margin of the opening, projects forwards and bends over the front of the cell, expanding into a variously-formed limb, and serving as protection to the mouth of the cell in front. ”

“ Just as in the case of the genital operculum the basal part is united in front to the appendages, the remainder being free, and separating the greater part of the two plate-like appendages. In the complete animal the median process of this segment is covered by the median process of the genital operculum. ”

“ The body is rather deep; the scales and cranial bones striated; the fins are large, many-rayed; their fulcra small; the dorsal is nearly opposite the interval between the ventrals and the anal; the base of the ventrals is long. The operculum is narrow and pointed below; a small triangular subopercular plate, whose anterior superior angle is produced upwards in a narrow linear process lying along the anterior margin of the operculum for some distance, is intercalated between the last-named bone and the anterior part of the upper margin of the interoperculum. ”

“ In such species as have a shelly operculum, the margin of the aperture is thickened all round, and has a parallel internal groove for its reception: the probable use of this groove I have detailed elsewhere. On the other hand, in those species which are known to have horny opercula, this margin and groove do not exist; and that part of the shell which is between the top of the aperture and the umbilicus, is thin and unprotected. This latter formation is by far the most frequent, and leads to the conclusion that the majority of these shells have their opercula horny. ”

“ A genus of land shells varying in shape from that of Pupa to that of a flat orb; the aperture is generally circular and the peritreme uninterrupted, thickened and sometimes reflected, the operculum is shelly and spiral. Two other genera of land shells are provided with opercula, and consequently might be confounded with this genus. In Helicina, the operculum is concentric and the peritreme is not continuous; while in the small genus hitherto almost unknown of Pupina, the peritreme is not continuous and there is a glassy enamel over the whole of the external surface. ”

“ In the former case, if an operculum (with which some species, both marine and land, close their mouths, more or less partially) exists, it will, generally in the case of land shells, remain in its place, adhering to the shell. In the latter, the decayed matter should be washed out, and the operculum, if any, replaced and fixed, say, on cotton filling the shell. This applies equally to land shells.
3. Care should be taken not to injure the edge or lip of the mouth of univalves, or the ligament of the hinge of bivalves. ”

“ The most interesting feature is the shape of the metastoma, which is roughly oval, with a narrow notch at the anterior and a broad base which is slightly cordate or emarginate (see fig. 98) . It measures 8.1 mm. in length and 7.2 mm. in greatest width (see fig. 95) . The female, or Type B (see fig. 97) , operculum is well preserved, showing even such details as the setae. The appendage is small, about one-third the length of the operculum, and consists of an undivided, rather broad lanceolate joint, with two triangular joints at the distal end. ”

“ The outer branchial rakers are long.
The narrow, slightly pitted, scaleless disk of the preoperculum bounds the scaly cheek behind and below, and has an entire edge with neither spine nor acute angle at the bend. The other pieces of the gill cover are closely covered with scales, only a little smaller than those of the body. The pretty wide thin inter-operculum lays freely over the gill membranes, and covers them when shut up. The sub-operculum is minutely crenulated on the edge, and has a small sub-membranous tip, which projects a little beyond the three opercular teeth. ”

“ Two pairs of membrane-closed vacuities separate the paired dermal bones of the cranial roof (possibly parietals and frontals) from the more laterally-placed post-temporals and squamosals, and there are no median plates posterior to the orbits, nor any representatives of supra-temporals. A feeble suboperculum is retained in addition to a small rayed operculum. Hyoidean hemibranch completely suppressed. Two genera only are known, each with a single species.
Fig. 289.—Polyodon folium. a, Anus; f, fulcra; n, nostrils; op, operculum; sc, rhombic scales on the upper caudal lobe; sp, left spiracle. ”

“ In a young Balanus, immediately after the metamorphosis, the operculum is attached by the opercular membrane all round to the summits of the compartments, and there cannot be said to be any orifice to the shell itself, but only an orifice or slit between the opercular valves; but during growth, as the compartments are added to at their basal edges, their upper ends are deserted, and cease to enclose the sack, within which lies the animal’s body. Hence the upper ends come to project freely, either quite separately as in some species of Pollicipes, where they cannot be said to form an orifice ”

“ The operculum is either horny, like that of the periwincle, or of a harder substance, like the shell itself.
"The umbilicus is a circular hole in the body of the shell. This perforation produces a very curious effect when it is very large. When the umbilicus is wanting, the shell is called imperforate.
"If the spire is truncated, it is decollated; if it is surrounded with spires, it is coronated, or crowned.
"Second section, without a regular spire. The teeth in univalve shells, as the cowry, are ridges upon the aperture. ”

“ The operculum always consists of more than one piece, generally of two pairs: they are either articulated to each other by serrated edges, and placed against each other conically, as in Balanus, or they lie flat in two pairs against each other. Through the ventral pair the cirrhi protrude.
The parietal valves, composing the principal part of the shell, vary in number, form and position. The anterior valves are placed on the same side with the cirrhi; the posterior, those on the opposite side; and those which remain between on each side are the lateral valves. ”

“ His body is whitish and soft, with a huge hump on the lower back armed with two small hooks. His enormous head is now seen to be apparently circular in outline, and we readily see how perfectly it would fill the opening of the burrow like an operculum. But a close examination shows us that this operculum is really composed of two halves, on two separate segments of the body, the segment at the extremity only being the true head, armed with its powerful, sharp, curved jaws. ”

“ The only difference between the genera Trochus and Turbo, as at present established, depends upon the calcareous or shelly, and the corneus or horny texture of the operculum.
CALCEOLA. Fam. Rudistes, Lam. and Bl.—Descr. Equilateral, inequivalve, triangular; umbones separated by a large triangular disc in the lower valve; cardinal margin straight, linear, dentated; lower valve large, deep; upper valve flat, semi-orbicular, forming a kind of operculum to the lower.—Obs. This singular shell, known only in a fossil state, in the Palæozöic beds, is placed by Linnæus in the genus Anomia. ”

“ Balanus balanoides and Chthamalus stellatus, I have observed that, when left uncovered by water, they kept the orifice of their operculums a little open, with a bubble of air within their sacks, so that the orifice was in fact closed by a thin septum of water, with air beneath; when disturbed, they closed their operculums with force, and expelled the bubble of air with a clicking noise, which has been noticed by Dr. Coldstream, [27] and has been thought to be made by the movement of the operculum itself. Bal. crenatus, a deep-water species, when out of water, keeps its operculum closed. ”

“ Continually enlarged by the animal, as the construction of its shell advances so as to fill up233 its mouth, the operculum measures the progressive widening of the spiral chamber by the progressive stages of its growth.
* * * * *
The animal, as he advances in the construction of his shell, increases continually his operculum, so as to adjust it to his mouth. He increases it, however, not by additions made at the same time all round its margin, but by additions made only on one side of it at once. ”

“ Fig. 281.—Musical apparatus of Cicada plebeia. (After Carlet.) A, Ventral view (Operculum on right side is removed) ; ap, apophysis; C, cavern; c, trochantin (cheville of Réaumur) ; ent, part of internal skeleton of abdomen; mi, specular membrane; m.pl, soft or folded membrane; P, base of leg; st, st′, st′′, stigmata; t, drum "timbale"; v, operculum; 1a, first, 2a, second abdominal segment: B, same seen laterally, portion of abdominal wall as well as operculum removed; A, point of insertion of hind wing; Mes, mesothorax; sc, scutum of metathorax; 3a, third abdominal segment; rest as in A.
Fam. ”

“ Here the supposition at once presents itself that the primitive tubicolar worm was a Protula,—that some of its descendants, which had already become developed into perfect Protulæ, subsequently improved themselves by the formation of an operculum which might protect their tubes from inimical intruders,—and that subsequent descendants of these latter finally lost the lateral filaments of the opercular peduncle, which they, like their ancestors, had developed. ”

“ The general appearance of the shell presents a medium between Phasianella and Littorina, but it is apparently destitute of an operculum.
LITTORINA. Fer. (Littus, the sea-shore.) Fam. Turbinacea, Lam.—Descr. Turbinated, thick; spire acuminated, consisting of few whorls, about one third of the axis in length; aperture entire, large, rounded anteriorly; outer lip thickened within, acute; columella rather flattened; operculum horny, spiral, with rapidly increasing volutions.—Obs. The shells composing this genus are known from Turbo and Phasianella by the horny operculum ”

“ Operaria: the workers in Hymenoptera.
Operative: in working order or actually working.
Opercula: two plates covering the vocal structure of Cicada, beneath.
Operculum: a lid or covering: in Diptera, the chitinous envelope covering the lower part of the muscid mouth; the labrum-epipharynx of Dimmock: the scutes covering the meso-thoracic stigmata: in Aleurodidae, the lid-like structure closing the vasiform orifice; q.v.
Ophthalmic: relating to the eye.
Ophthalmotheca: that part of the pupa that covers the eyes.
Opisthogoneate: having the organs of generation at hind end of body. ”

“ There is an articulary cavity at its upper anterior angle for its junction with the hyomandibular. The oblong lamella below the operculum is the suboperculum (32) , and the one in front of this latter, below the horizontal limb of the præoperculum, is the interoperculum (33) , which is connected by ligament with the angular piece of the lower jaw, and is also attached to the outer face of the hyoid, so that the gill-covers cannot open or shut without the hyoid apparatus executing a corresponding movement. ”

“ The aperture of many species of shells remains constantly open; but in a great number of species it is occasionally closed, whenever the animal is retracted within the shell, by a calcareous or horny piece called the operculum. ”

“ An unguiculated operculum is one in which the layers are disposed laterally, and the nucleus constitutes part of the outer edge.
UNGULINA. Daud. (Ungula, a nail or claw.) Fam. Mactracea, Lam. Conchacea, Bl.—Descr. Equivalve, sub-orbicular, sub-equilateral, with margins entire, simple, closed all round; hinge with one short, sub-divided cardinal tooth in each valve, and a very minute additional tooth in one valve, an oblong ligamentary pit divided into two portions, one of which receives the cartilage, the external ligament is immediately below the umbones ”

“ Nasal operculum very narrow anteriorly, nude for anterior and exterior portion, the frontal feathering extending anteriorly much beyond middle of nasal operculum, forming a more or less distinct but sometimes very short and obtuse point or antia on each side of the mesorhinium. ”

“ A small curved notch marks the separation between the interoperculum and sub-operculum.
The scales extend on the crown of the head to the middle of the orbits. The snout, lips, jaws, the place at the corner of the mouth over which the maxillary glides and the gill membrane are scaleless. The scales of the body are very regularly disposed, showing rhomboidal disks when in situ, with strongly ciliated edges. The lateral line ascends at its commencement and bends rather suddenly under the first soft dorsal ray to run near and parallel to the ridge of the back. ”