Coelom sentence example
The body wall of the Chaetopoda consists of a "dermo-muscular" tube which is separated from the gut by the coelom and its peritoneal walls, except in most leeches.
Several forms of cells float freely in the fluid of the coelom.
In another sense also the coelom is not a closed cavity, for it communicates in several ways with the external medium.
Thus the nephridia, in this case at least, are a part of the coelom and are not shut off from it by a layer of peritoneum, as are other organs which lie in it, e.g.
It is therefore a haemocoel, the coelom of the developed insect being represented only by the cavities of the genital glands and their ducts.Advertisement
With a few exceptions among the Polychaeta the vascular system is always present among the Chaetopoda, and always consists of a system of vessels with definite walls, which rarely communicate with the coelom.
In this worm the ventral blood-vessel is so swollen as to occupy nearly the whole of the available coelom.
The cavity of the former has nothing to do with coelom.
The cavity of the latter is coelom.
The nephridia in this group are invariably coiled tubes with an intracellular lumen and nearly invariably open into the coelom by a funnel.Advertisement
We have thus the replacement of a spermatheca, corresponding to those of the remaining families of Oligochaeta, and derived, as is believed, from the epidermis, by a structure performing the same function, but derived from the mesoblastic tissues, and with a cavity which is coelom.
Nerve cord lies in coelom; brain in first segment or prostomium in many forms. Clitellum generally only two or three segments and more anterior in position than in Terricolae.
The ganglia are crowded at the posterior end of the body as in leeches, and there is much tendency to the obliteration of the coelom as in that group. Pterodrilus and Cirrodrilus bear a few, or circles of, external processes which may be branchiae; Bdellodrilus and Astacobdella have none.
Coelom generally reduced to a system of tubes, sometimes communicating with vascular system; in Acanthobdella and Ozobranchus a series of metamerically arranged chambers as in Oligochaeta.
This coelom is lined by peritoneal cells and is divided into a series of metameres by septa which correspond to the segmentation of the FIG 15.Advertisement
In Acanthobdella the testes are, however, not contained in the general coelom, and the nephridia lie in the septa.
It is remarkable, in view of the spaciousness of the coelom, that the funnels of the latter have not been seen.
Ozobranchus possesses a coelom which is less typically chaetopodous than that of Acanthobdella, but more so than in other leeches.
These regions of the coelom end at the ends of the body and communicate with each other by means of a branched system of coelomic sinuses, which are in places very fine tubes.
Neither in this genus nor in the last is there any communication between coelom and vascular system.Advertisement
In Clepsine (Glossiphonia) there is a further breaking up of the coelom.
Here also the blood system has no communication with the sinus system of the coelom.
Goodrich that the vascular system and the coelom are in communication (as in vertebrates by means of the lymph system).
On the other hand, it has been held that in these leeches there is no vascular system at all and that the entire system of spaces is coelom.
These tubes are lined by flattened epithelium and often contain blood capillaries; they communicate with the coelom and are to be regarded as prolongation of it into the thickness of the body wall.Advertisement
The gonads and their ducts in the Hirudinea invariably form a closed system of cavities entirely shut off from the coelom in which they lie.
The ovaries are solid bodies, of which the outer layer becomes separated from the plug of cells lying within; thus a cavity is formed which is clearly coelom.
The existence of two renal organs in Patella, and their relation to the pericardium (a portion of the coelom), is important.
It has been pointed out that the cavity of the sacs corresponds in many particulars with the coelom of higher animals, and in Lebidinsky's observations on the development there is some support to the view that a coelom exists.
There are no specialized sense-organs or vascular or respiratory systems. There is a wide body-cavity, but as this has no connexion with the renal or reproductive organs it cannot be regarded as a coelom, but probably is a blood-space or haemocoel.
Of these divisions of the coelom the first two communicate with the exterior by means of a pair of ciliated pore-canals placed at the posterior end of their respective segments.
There are reasons for supposing that the truncal coelom was at one time provided with pore-canals, but supposed vestiges of these structures have only been described for one genus, Spengelia,in which they lie near the anterior end of the truncal coelom.
In the direct development Bateson showed that the three divisions of the coelom arise as pouches constricted off from the archenteron or primitive gut, thus resembling the development of the mesoblastic somites of Amphioxus.
In Crania it is completely shut off from the main coelom, but in Lingula it communicates freely with this cavity.
They contain the same fluid as the general coelom.
The stalk is an extension of the ventral body-wall, and contains a portion of the coelom which, in Discinisca and Lingula, remains in communication with the general body cavity.
The ovary and testes are heaped-up masses of red or yellow cells due to a proliferation of the cells lining the coelom.
Both belong to the category of " coelomoducts," namely, r- ' tubular or funnel-like portions of the coelom opening to the exterior in pairs in each somite (potentially,) and usually persisting in only a few somites as either "urocoels" (renal organs) or "gonocoels"(genital tubes).
The gonadial tubes (gonocoels or gonadial coelom) are originally reticular and paired, though they may be reduced to a simpler condition.
Of these latter, two grades were further distinguished by Lankester - those which remain possessed of a single archenteric cavity and of two primary cell-layers (the Coelentera or Diploblastica), and those which by nipping off the archenteron give rise to two cavities, the coelom or body-cavity and the metenteron or gut (Coelomata or Triploblastica).
The tracing of the exact mode of development, cell by cell, of the diblastula, the coelom, and the various tissues of examples of all classes of animals was in later years pursued with immense activity and increasing instrumental facilities.
Archigetes appendiculatus lives throughout life in the coelom of Tubifex and of Limnodrilus.
Injection of the fluid-extract of such worms into the blood or coelom of their host causes grave disturbance.
Annelids are segmented worms, and differ from the Arthropoda, which they closely resemble in many respects, by the possession of a portion of the coelom traversed by the alimentary canal.
It, as in other Mollusca, is not a blood-space but develops from the coelom, and it communicates with the exterior by the pair of renal tubes.
True nephridia do not primarily open into the coelom, as was formerly taught, but are intra-cellular ducts in the mesoderm.
The coelom opens to the exterior by ducts which are primarily genital ducts by which the ova or sperms are discharged.
In Mollusca the coelom is reduced and consists of two parts, the pericardial cavity which surrounds the heart, and the cavity of the gonads or generative organs.
In the former the genital coelom and the pericardial coelom are continuous and the reproductive cells escape by the renal ducts.
The coelom is formed as a cavity or cavities in the interior of these cell-masses.
In some cases the coelom is formed as a single cavity, and renal and generative organs are formed from its walls.
The renal organs are tubular outgrowths of the pericardial parts of the coelom; the reproductive cells are derived from cells lining the generative portion.
They agree with primitive nephridia in being of ectodermic origin, in consisting of perforated cells in linear series, and in having no communication with the coelom.
The Prorhipidoglossomorpha are distinguished by the separation of the genital coelom from the pericardium, and by the long visceral commissure passing ventral to the intestine.
The Cephalopoda can be derived without much difficulty from the schematic Mollusc, if we assume that some metameric repetition of organs has occurred, as explained above in reference to the coelom.
It is characteristic of the group that the mouth should be the only means of ingress to and egress from the digestive sac and that no true perivisceral space or coelom exists in the sense in which these terms are used in higher Invertebrates.
The two lateral lobes contain the coelom; each separates off in front a segment which forms the head and presumably then divides again to form anteriorly the trunk, and posteriorly the tail regions.
As in other molluscs the coelom is represented by a large pericardial cavity, situated above the intestine posteriorly, and a generative sac which is single and median and situated in front of the pericardium, except in the Nuttalochiton hyadesi, where the gonads are in a similar position, but are paired.
The coelom differs from that of the Chitons in the fact that the cavities of the genital organs are continuous with it, and in the fact that there is only one pair of coelomoducts resembling the renal organs of Chitons, but serving also as genital ducts.
The chief difference is that the gonad or generative portion of the coelom is single and median, opening into the pericardium by a single posterior aperture.
There should also be only a single coelom, or a pair of lateral coelomic cavities.
On this view then the Aplacophora are more primitive than the Polyplacophora in the relations of coelom, gonad and coelomoducts; and the genital ducts of the Chitons have arisen either by metameric repetition within the group, or by the gradual loss of an original connexion between the generative sac and the renal tube, as in Lamellibranchs and Gastropods, the generative sac acquiring a separate duct and opening to the exterior on each side.
The body is composed of a large number of segments; the prostomium bears a pair of tentacles; the nervous system consists of a brain and longitudinal ventral nerve cords closely connected with the epidermis (without distinct ganglia), widely separated in Saccocirrus, closely approximated in Protodrilus, fused together in Polygordius; the coelom is well developed, the septa are distinct, and the dorsal and ventral longitudinal mesenteries are complete; the nephridia are simple, and open into the coelom.
Polygordius differs from Protodrilus and Saccocirrus in the absence of a distinct suboesophageal muscular pouch, and in the absence of a peculiar closed cavity in the head region, which is especially well developed in Saccocirrus, and probably represents the specialized coelom of the first segment.
These are suspended to the muscular bodywall by a double membrane, called the ligamentum denticulatum, which forms at once the roof of the atrial chamber and the floor of a persistent portion of the original body-cavity or coelom (the dorsal coelomic canal on each side of the pharynx).
At the top of each of these pouches there is a minute orifice, the aperture of a small tubule lying above each pouch in the dorsal coelom.
They communicate with the coelom by several openings or nephrostomes, and with the atrium by a single opening in each case, the nephridiopore.
At the level of its posterior end a pair of funnel-shaped pouches of the atrium are produced forwards into the dorsal coelom.
These spaces make up the apparent body-cavity, the ta, Stomach of common crab, true body-cavity or coelom having Cancer pagurus, laid open, been, for the most part, obliter showing b, b, b, some of the ated by the great expansion of calcareous plates inserted in the blood-containing spaces.
Development shows that the glandular tube is mesoblastic in origin and is of the nature of a coelomoduct, while the end-sac is to be regarded as a vestigial portion of the coelom.
The dorsoventral and the parapodial muscles are much developed, whilst the coelom is reduced mostly to branched spaces in which the genital products ripen, Full-grown myzostomids are hermaphrodite.
Into this same cloacal chamber open ventrally a pair of ciliated tubes communicating by funnels with the coelom (Nansen and Wheeler); these are possibly nephridia, and excretory in function.
We may, with Sedgwick, suppose the coelom to have originated by the enlargement and separation of pouches that pressed outwards from the archenteron into the thickened body-wall (such structures as the genital pouches of some Coelentera, not yet shut off from the rest of the cavity), and they would probably have been four in number and radially disposed about the central cavity.
Into the space between the walls of the coelom and the outer body-wall, originally filled with jelly, definite cells now wandered, chiefly derived from the coelomic walls.
The forward portion of the anterior coelom shared in the constriction and elongation of the preoral lobe; but its hinder portion was dragged up along with the water-pore and formed a canal lying moccth ive along the outer wall (the gerei tatpore parietal canal).
Since this must have, on our theory, enclosed the parietal canal from the anterior coelom, it is possible that the genital products were developed from the lining cells of that cavity, and that the genital pore was nothing but its original pore not yet united with that from the water-sac. The concrescence of these pores can be traced in other cystids; but as the genital organs became affected by radial symmetry the original function of the duct was lost, and the reproductive elements escaped to the exterior in another way.
The remains of the original genital gland within the theca became the "axial organ" surrounded by the "axial sinus" derived from the anterior coelom, and this again by structures derived from the right posterior coelom, which, as explained above, had been depressed to the aboral pole.
The generative organs arid coelom probably did not send extensions along the rays into the brachioles; but apparently nerves from the aboral centre, after passing through the thecal plates, met in a circumoral ring, from which branches passed into the plate under each main food-groove, and thence supplied the brachioles.
Pelmatozoa in which epithecal extensions of the food-grooves, ambulacrals, superficial oral nervous system, blood-vascular and water-vascular systems, coelom and genital system are continued exothecally upon jointed outgrowths of the abactinal thecal plates (brachia), carrying with them extensions of the abactinal nerve-system.
I, Coelom of the first somite It also shows us that the neurowhich carries the anten meres, no less than the embryonic nae and is in front of the coelomic cavities, point to the existmouth.
D, is the neuromere of the second III and IV, Coelom of the-third or buccal somite.
The praeseptal or lophophoral coelom is continued into each of the tentacles and into the (After Allman.) FIG.
The postseptal coelom is partially divided by a ventral mesentery which is attached along the entire length of the convex.
Each nephridium is provided with either one or two funnels which open into the postseptal division of the coelom (ne.f).
One of the most formidable difficulties in the way of the attempt to reduce Actinotrocha to the Pterobranchiate type of structure is the condition of the coelom in the former.
The epistome of the adult Phoronis cannot well be the proboscis since its cavity is continuous with the lophophoral coelom, and because the praeoral hood of Actinotrocha is entirely lost at the metamorphosis.
It thus appears that in Peripatus the coelom does not develop a perivisceral portion, but gives rise only to the renal and reproductive organs.
Peripatus is an Arthropod, as shown by (1) the presence of appendages modified as jaws; (2) the presence of paired lateral ostia perforating the wall of heart and putting its cavity in communication with the pericardium; (3) the presence of a vascular body cavity and pericardium (haemocoelic body cavity); (4) absence of a Derivisceral section of the coelom.
The coelom is represented as surrounded by a thick black line, except in the part which forms the internal vesicle of the nephridium.
Expansion of the extra-embryonic coelom cavity allows the yolk sac and allantois to expand into the coelom cavity allows the yolk sac and allantois to expand into the coelom cavity from the gut of the embryo.
In the Polychaeta, which are to be regarded as structurally simpler forms than the two groups just referred to, there is but little subdivision of the coelom of the segments, indeed a tendency in the reverse direction, owing to the suppression of septa.
It is therefore by no means certain that so profound a difference embryologically can be asserted to exist between the excretory nephridia and the ducts leading from the coelom to the exterior, which are usually associated with the extrusion of the genital products among the Chaetopoda.
It might be well to term these structures, mostly serving as gonad ducts, which have an undoubted resemblance to nephridia, and for the most part an undoubted connexion with nephridia, "Nephro dinia," to distinguish them from another category of "ducts" which are communications between the coelom and the exterior,.
Furthermore, in the mid region of the body this coelom is broken up by metamerically arranged septa, as in Acanthobdella.
Some of the endothelial cells lining the coelom are ciliated, the cilia keeping the corpusculated fluid contents in movement.
The superior surface is in contact with the diaphragm, but has peritoneum between (see Coelom And Serous Membranes).
Thus, about 1875, the distinction of Echinoderms from such radiate animals as jelly-fish and corals (see Coelentera), by their possession of a body-cavity ("coelom") distinct from the gut, was fully realized; while their severance from the worms (especially Gephyrea), with which some Echinoderms were long confused, had been necessitated by the recognition in all of a radial symmetry, impressed on the original bilateral symmetry of the larva through the growth of a special division of the coelom, known as the "hydrocoel," and giving rise to a set of water-bearing canals - the watervascular or ambulacral system.
Its cavity also is at first independent of the coelom though later invaded by the latter.
The Chaetopoda are characterized by a spacious coelom, which is divided into a series of chambers in accordance with the general metamerism of the body.
This is the typical arrangement, which is exhibited in the majority of the Polychaeta and Oligochaeta; in these the successive chambers of the coelom are separated by the intersegmental septa, sheets of muscle fibres extending from the body wall to the gut and thus forming partitions across the body.
The coelom is lined throughout by cells, which upon the intestine become large and loaded with excretory granules, and are known as chloragogen cells.
In these and other Chaetopods the coelom is also put into indirect relations with the outside world by the nephridia and by the gonad ducts.
In these features, and in the fact that the gonads are local proliferations of the coelomic epithelium, which have undergone no further changes in the simpler forms, the coelom of this group shows in a particularly clear fashion the general characters of the coelom in the higher Metazoa.
It has been indeed largely upon the conditions characterizing the Chaetopoda that the conception of the coelom in the Coelomocoela has been based.
Among the simpler Chaetopoda the coelom retains the character of a series of paired chambers, showing the above relations to the exterior and to the gonads.
To this pericardial coelom is frequently added a gonocoel enclosing the gonads and the funnels of their ducts.
The division and, indeed, partial suppression of the coelom culminates in the leeches, which in this, as in some other respects, are the most modified of Annelids.
In others, it lies in the coelom, often surrounded by a special and occasionally rather thick sheath.
In addition to the coelom, another system of fluid-holding spaces lies between the body wall and the gut in the Chaetopoda.
It has been asserted (and denied) that the cellular rod which is known as the "Heart-body" (Herzkorper), and is to be found in the dorsal vessel of many Oligochaeta and Polychaeta, is formed of cells which are continuous with the chloragogen cells, thus implying the existence of apertures of communication with the coelom.
This state of affairs has no antecedent improbability about it, since in the Vertebrata the coelom is unquestionably confluent with the haemal system through the lymphatic vessels.
Carry the process but a little farther and the coelom disappears and its place is taken by a blood space or haemocoel.
It has been held that the condition shown in certain leeches tend to prove that the coelom and haemocoel are primitively one series of spaces which have been gradually differentiated.
Ray Lankester to the members of a series of tubes, proved in some cases to be excretory in nature, which exist typically to the number of a single pair in most of the segments of the Chaetopod body, and open each by a ciliated orifice into the coelom on the one hand, and by a pore on to the exterior of the body on the other.
Rarely the nephridium does not communicate with the coelom; in such cases the nephridium ends in a single cell, like the "flame cell" of a Platyhelminth worm, in which there is a lumen blocked at the coelomic end by a tuft of fine cilia projecting into the lumen.
In all these cases we have a duct which has a usually wide, always intercellular, lumen, generally, if not always, ciliated, which opens directly into the coelom on the one hand and on to the exterior of the body on the other.
Both series of organs consist essentially of a ciliated tube leading from the coelom to the exterior.
In both the cavity originally or immediately continuous with the coelom appears first in the funnel and grows backwards.
As an example of the former it has been shown (Beddard) that a large median sac in Lybiodrilus is at first freely open to the coelom, that it later becomes shut off from the same, that it then acquires an external orifice, and, finally, that it encloses the ovary or ovaries, between which and the exterior a passage is thus effected.
Beneath this again is a distinct peritoneum lining the coelom, which appears to be wanting as a special layer in some Polychaetes (Benham, Gilson).
In the Oligochaeta there is a closer correspondence between external metamerism and the divisions of the coelom than is apparent in some Chaetopods.