68, k), absent in Chondrophorida and Cystophorida; they are contractile and resemble, both in appearance, structure and function, the umbrella of a medusa, with radial canals, ring-canal and velum; but they are without manubrium, tentacles or sense-organs, and are always bilaterally symmetrical, a peculiarity of form related with the fact that they are attached on one side to the stem.
The sub-umbrella invariably shows a velum as an inwardly projecting ridge or rim at its margin, within the circle .of tentacles; hence the medusae of this sub-class are termed craspedote.
corresponding to the twelve Velum.
external epithelium is flat on the ex-umbral surface, more columnar on the sub-umbral surface, where it forms the muscular tissue of the sub-umbrella and the velum.
The muscle-fibres arise as processes from the bases of the epithelial cells; such cells may individually become sub-epithelial in position, as in the polyp; or, in places where muscular tissue is greatly developed, as in the velum or sub-umbrella, the entire muscular epithelium may be thrown into folds in order to increase its surface, so that a deeper sub-epithelial muscular layer becomes separated completely from a more superficial bodyepithelium.
The circular system is developed continuously over the entire subumbral surface, and the velum represents a special local development of this system, at a region where it is able to act at the greatest mechanical advantage in producing the contractions of the umbrella by which the animal progresses.
The nervous system of the medusa consists of sub-epithelial ganglion-cells, which form, in the first place, a diffuse plexus of nervous tissue, as in the polyp, but developed chiefly on the subumbral surface; and which are concentrated, in the second place, to form a definite central nervous system, never found in the polyp. In Hydromedusae the central nervous system forms two concentric nerverings at the margin of the umbrella, near the base of the velum.
One, the " upper " or ex-umbral nervering, is derived from the ectoderm on the ex-umbral side of the velum; it is the larger of the two rings, containing more numerous but smaller ganglioncells, and innervates the tentacles.
29, 30) as an open pit at the base of the velum, on its subumbral side.
We then find the typical otocyst of the Leptomedusae, a vesicle bulging on the ex-umbral side of the velum (figs.
40); in Geryonia, however, it remains double, and the centripetal canals arise by parting of the two layers; (4) excretory endoderm, lining pores at the margin of the umbrella, occurring in certain Leptomedusae as socalled " marginal tubercles," opening, on the one hand, into the ring-canal and, on the other hand, to the exterior by " marginal funnels," which debouch into the sub-umbral cavity above the velum.
Here the medusoid, attached by the centre of its ex-umbral surface, has lost its velum and sub-umbral muscles, its sense organs and mouth, though still retaining rudimentary tentacles.
stage the mouth is D, Sketch of exter- v, Velum.
43, A); in this the mouth is formed distally as a perforation (B); next the sides of the tube so formed bulge out laterally near the attachment to form the umbrella, while the distal undilated portion of the tube represents the manubrium (C); the umbrella now grows out into a number of lobes or lappets, and the tentacles and tentaculocysts grow out, the former in a notch between two lappets, the latter on the apex of each lappet (D, E); finally, the velum arises as a growth of the ectoderm alone, the whole bud shapes itself, so to speak, and the little medusa is separated off by rupture of the thin stalk connecting it with the parent (F).
s.c, Cavity of ento- v, Velum.
44, F) grow out from the ring-canal, and the double plate of ectoderm on the distal side of the entocodon becomes perforated, leaving a circular rim composed of two layers of ectoderm, the velum (v) of the medusa.
It is seen from the foregoing account of medusa - budding that the entocodon is a very important constituent of the bud, furnishing some of the most essential portions of the medusa; its cavity becomes the subumbral cavity, and its lining furnishes the ectodermal epithelium of the manubrium and of the sub-umbral cavity as far as the edge of the velum.
D, E, F, a thickening of the ecto- v, Velum.
Ve, The velum.
The manubrium bearing the gonads is mouthless, and the umbrella is without tentacles, sense-organs, velum or radial canals.
The communication with the atrium is guarded by a valvula cardiaca dextra, which only in function represents the mammalian tricuspid; it consists of an oblique reduplication of the muscular fibres together with the endocardiac lining of the right ventricle, while the opposite wall is convex and forms neither a velum nor papillary muscles, nor chordae tendineae.
y e, Velum.
The preconchylian invagination or shell-gland is formed in the embryo behind the velum, on the surface opposite the blastopore.
In forms which are naked in the adult state, the shell falls off soon after the reduction of the velum, but in Cenia, Runcina and Vaginula the shell-gland and shell are not developed, and the young animal when hatched has already the naked form of the adult.
vr, Velum or circlet of ciliated cells.
Foot divided into two, posterior half bearing the operculum; a wide epipodial velum; shell turbinated.
36) to the adult form has not been properly observed, and many interesting points as to the true nature of folds (whether parapodia or mantle or velum) have yet to be cleared up by a knowledge of such development in forms like Tethys, Doris, Phyllidia, &c. As in other Molluscan groups, we find even in closely-allied genera (for instance, in Aplysia and Pleurobranchidium, and other genera), the f operculum.
Head broad, surrounded by a funnel-shaped velum or hood; no radula; dorsal appendages foliaceous.
derm is coincident with the v, v, The ciliated band marking hindmost extremity of the out the velum.
There is a veliger stage in development, but the velum is reduced.
As a decoration, rather than for practical reasons, a fine folded cloth (pannisellus, sudarium, velum, Eng.
a, the annulus, or remnant of C, longitudinal section of mature velum partiale.
v, remains of volva or velum p, the pileus.
The larva for a time swims freely in the sea-water, having a circlet of cilia round the body in front of the mouth, forming the velum.
The shell is developed on the dorsal surface behind the velum, the foot on the opposite or ventral surface behind the mouth.
subumbral muscles form a rim known as the velum (v., see fig.
divisions of the medusae are characterized by the presence or absence of a velum.
Correlated with the well-developed muscular system and sense-organs of the medusa, we find also a distinct nervous system, either, when there is no velum, in the form of concentrations of nervous matter in the vicinity of each sense-organ, or, when a velum is present, as two continuous rings running round the margin of the umbrella, one external to the velum (exumbral nerve-ring, n.r l, see fig.
The exumbral nerve-ring is the larger and supplies the tentacles; the subumbral ring supplies the velum.
The test is really a ciliated velum developed in the normal position at the apical pole but reflected backwards in such a way as to cover the original ectoderm except at the posterior end.
A similar modification of the velum occurs in Dentalium and in Myzomenia among the Amphineura.
It is termed the " velum," and is frequently drawn out (From Balfour, after Bobretzky.) FIG.
There is a single pair of these organs situated immediately behind the velum.
velum and the great paired op, Operculum.
Later the ciliated ring or velum disappears and seven imbricated calcareous plates, made up of flattened spicules, are formed on the dorsal surface.
C, Longitudinal section of mature v, Remains of volva or velum p, The pileus.
The mouth consists of two portions, an outer vestibule and an inner apertura oris; the latter is surrounded by a sphincter muscle, which forms the so-called velum.
The velum is also provided with a circlet of twelve tantacles (in some species sixteen) which hang backwards into the pharynx; these are the velar tentacles.
The medusoids have a muscular velum of ectoderm and mesogloea only.
The medusoids have no true velum; in some cases a structure more or less resembling this organ, termed a velarium, is present, permeated by endodermal canals.
The umbrella has a lobed, indented margin, a character only seen amongst Hydromedusae in the order Narcomedusae, and it is without the characteristic velum of the Hydromedusae; hence the Scyphomedusae are sometimes termed Hydrozoa Acraspeda.
A true velum, such as is found in Hydromedusae, never contains endoderm.
The velum is peculiar, being reflected backwards over the body and bearing, besides an apical tuft, three or four rings of cilia.
The foot arises as a prominence on the ventral surface and grows forward, and at the end of five or six days the velum atrophies and the foot becomes the organ of locomotion; the animal then ceases to swim and sinks to the bottom.
The Rotifera are characterized by the retention of what appears in Molluscs and Chaetopods as an embryonic organ, the velum or ciliated prae-oral girdle, as a locomotor and foodseizing apparatus, and by the reduction of the muscular parapodia to a rudimentary or non-existent condition in all present surviving forms except Pedalion.
The palate is long and narrow; its mucous surface has seventeen pairs of not very sharply defined oblique ridges, extending as far back as the last molar tooth, beyond which the velum palati extends for about 3 in., having a soft corrugated surface, and ending posteriorly in an arched border without a uvula.
Under ordinary circumstances the horse breathes entirely by the nasal passages, the communication between the larynx and the mouth being closed by the velum palati.
v, Velum or integument.
The range over which the velum opens depends on when the previous obstruent was, and whether it was nasal.
velar articulation - we do this with the back of the tongue against the velum.
Non-nasal-CNS | nasal-CNS The velum starts opening 20ms before the start of the nasal, finishes 10ms after the start of the nasal.
closing the velum The velum is always fully open at the end of a nasal CNS.
opening the velum The velum is always fully open at the end of a nasal CNS.
The tentacles are based on a muscular band called the velum.
(I) The polyp, when present, is without the strongly developed longitudinal retractor muscles, forming ridges (taeniolae) projecting into the digestive cavity, seen in the scyphistoma or scyphopolyp. (2) The medusa, when' present, has a velum and is hence said to be craspedote; the nervous system forms two continuous rings running above and below the velum; the margin of the umbrella is not lobed (except in Narcomedusae) but entire; there are characteristic differences in the sense-organs (see below, and Scyphomedusae); and gastral filaments (phacellae), subgenital pits, &c., are absent.
The other, the " lower " or subumbral nerve-ring, is derived from the ectoderm on the sub-umbral side of the velum; it contains fewer but larger ganglion-cells and innervates the muscles of the velum (see diagram in article Medusae).
The pit has its opening turned towards the sub-umbral cavity, while it base or fundus forms a bulge, more or less pronounced, on the ex-umbral side of the velum.
The Leptomedusa in form is generally shallow, more or less saucer-like, with velum less developed than in Anthomedusae (fig.
B, Trochosphere of an Opisthobranch (Pleurobranchidium) showing - shgr, the shell-gland or primitive shell-sac; v, the cilia of the velum; ph, the commencing stomodaeum or oral invagination; ot, the left otocyst; pg, red-coloured pigment spot.
m, Mouth; f, foot; v, velar area, the margin v corresponding with the ciliated band which demarcates the velar area or velum of the embryo Gastropod (see fig.
A true velum is absent, as already stated, but in Charybdaea (fig.
Velar articulation - we do this with the back of the tongue against the velum.
Closing the velum The velum is always fully open at the end of a nasal CNS.
Opening the velum The velum is always fully open at the end of a nasal CNS.
Beyond that, construction paper, velum, parchment, even transparent sheets will work.
These monogrammed velum paper cones hold soft petals for tossing at the bride and groom as they depart.
If you're looking for a retro style luggage set with just a touch of pink, Rupert Sanderson's unique cream velum retro luggage might be what you're looking for.
velum or pre-oral (cephalic) lobe ever develops.
The word usage examples above have been gathered from various sources to reflect current and historial usage. They do not represent the opinions of YourDictionary.com.