Its cavity also is at first independent of the coelom though later invaded by the latter.
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.
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 another sense also the coelom is not a closed cavity, for it communicates in several ways with the external medium.
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.
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.
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.
Goodrich), that among the Hirudinea the coelom, which is largely broken up into narrow tubes, may be confluent with the tubes of the vascular system.
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.
The Capitellidae, in which the vascular system has vanished altogether, leaving a coelom containing haemoglobin-impregnated corpuscles.
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.
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.
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.
- Diagrams of various Earth of several Geoscolicidae, the nephridiopores indicate the segments; to each segment corresponds internally a chamber of the coelom which is separated from adjacent segments by transverse septa,which are only unrecognizable in the genus Aeolosoma and in the head region of other Oligochaeta.
The nephridia in this group are invariably coiled tubes with an intracellular lumen and nearly invariably open into the coelom by a funnel.
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.
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.
The coelom of the Hirudinea differs in most genera from that of the Oligochaeta and Polychaeta.
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.
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.
Goodrich that the vascular system and the coelom are in communication (as in vertebrates by means of the lymph system).
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.
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 existence of two renal organs in Patella, and their relation to the pericardium (a portion of the coelom), is important.
Mesoderm, Coelom and Blood-System.
It is therefore a haemocoel, the coelom of the developed insect being represented only by the cavities of the genital glands and their ducts.
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.