In any case the cavity of the prostomium is single, and not formed, as is the cavity of the segments of the body, by paired coelomic chambers.
Thus in the Chaetopoda the perivisceral cavity is coelomic; in this respect the group contrasts with the Arthropoda and Molluscs, where the perivisceral cavity is, mainly at least, part of the vascular or haemal system, and agrees with the Vertebrata.
Among the Oligochaeta the dorsal vessel in Dinodrilus and Megascolides is enclosed in a separate coelomic chamber which may or may not communicate with the main coelomic cavity.
The oviduct of Lumbricus) are parts of the coelomic wall itself, which have grown out to the exterior.
The nephridia, in fact, on this view, are ectodermic ingrowths, the coelomoducts coelomic outgrowths.
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.
More usually, and indeed in nearly every other case among the Oligochaeta and Hirudinea, the coelomic aperture of the nephridium consists of several cells, ciliated like the nephridium itself for a greater or less extent, forming a funnel.
There is no possibility that sperm and ova can escape by these tubes not in company with coelomic fluid.
Quite independent of these are certain large dorsally situate funnel-like folds of the coelomic epithelium, ciliated, but of which no duct has been discovered leading to the exterior.
These cavities communicate with the exterior through the gonad ducts, which have nothing to do with them, but whose coelomic funnels are taken up by them in the course of their growth.
They are essentially spherical, pear-shaped or oval sacs opening on to the exterior but closed at the coelomic end.
In Heliodrilus the blind extremity of the spermatheca is enclosed in a coelomic sac which is in connexion with the sacs envolving the ovaries and oviducts.
Moreover, upon the intestine the coelomic cells are modified into chloragogen cells.
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.
The gut has no coelomic space surrounding it.
In favour of regarding the vascular system as totally absent, is the fact that the median coelomic channels contain no dorsal and ventral vessel.
A further fact must be considered in deciding this question, which is the discovery of ramifying coelomic tubes, approaching close to, but not entering, the epidermis in the Polychaete Arenicola.
Burger) in coelomic spaces close to nephridial funnels, which have, however, no relation to the gonad ducts.
The large sacs which have been termed vagina are suggestive of the large coelomic spermathecae in Eudrilids, a comparison which needs, however, embryological data, not at present forthcoming, for its justification.
No doubt can be entertained that the greater part of the inner layer corresponds to the mesoderm of more ordinary embryos, for the coelomic pouches, the germ-cells, the musculature and the vascular system all arise from it.
Montgomery has also described certain spaces which may be coelomic lying between the alimentary canal and the inner longitudinal layer of muscles in the Heteronemertini.
In 1883-1886 Bateson showed by his embryological researches that the Enteropneusta exhibit chordate (vertebrate) affinities in respect of the coelomic, skeletal and nervous systems as well as in regard to the respiratory system, and, further, that the gill-slits are formed upon a plan similar to that of the gillslits of Amphioxus, being subdivided by tongue-bars which depend from the dorsal borders of the slits.
Coelomic space in Balano- vv, vessel.
It seems likely that the coelomic pore-canals were originally excretory organs, but in the existing Enteropneusta the pore-canals (especially the collar canals) have, as we have seen, acquired new functions or become vestigial, and the function of excretion is now mainly accomplished by a structure peculiar to the Enteropneusta called the glomerulus, a vascular complex placed on either side of the anterior portion of the stomochord, projecting into the proboscis-coelom.
These often have the form of prisms of calcite surrounded by a cuti cular meshwork; the whole is nourished and kept alive by processes, which in Crania are branched; these perforate the shell and permit the access of the coelomic fluid throughout its substance.
Besides this main coelomic cavity there are certain other spaces which F.
Blochmann regards as coelomic, but it must be remembered that his interpretation rests largely on histological grounds, and at present embryological confirmation is wanting.
These sinuses are completely shut off from all other cavities, they do not open into the main coelomic space nor into the small arm-sinus, nor does the right sinus communicate with the left.
The posterior occlusor muscles lie in a special closed space which Blochmann also regards as coelomic. The posterior end of the intestine is similarly surrounded by a closed coelomic space known as the peri-anal sinus in which the rectum lies freely, unsupported by mesenteries.
There is further a great tendency for the endothelial cells to form muscles, and this is especially pronounced in the small arm-sinus, where a conspicuous muscle is built up. The mantle-sinuses which form the chief spaces in the mantle are diverticula of the main coelomic cavity.
Previously to this, Lankester's pupil Gulland had shown (1885) that in the embryo the coxal gland is a comparatively simple tube, which opens to the exterior in this position and by its other extremity into a coelomic space.
In the Phylactolaemata the outermost layer of the bodywall is a flexible, uncalcified cuticle or "ectocyst," beneath which follow in succession the ectoderm, the muscular layers and the coelomic epithelium.
Even the ectoderm can rarely be recognized as an obvious epithelium except in regions where budding is taking place, while muscular layers are always absent and a coelomic epithelium can seldom be observed.
Recent research, however, points to these cords of the rete testis et ovarii as being derived from the coelomic epithelium instead of from the mesonephros.
As each genital gland enlarges it remains attached to the rest of the intermediate cell mass by a constricted fold of the coelomic membrane, known as the mesorchium in the male, and the mesovarium in the female.
From the coelomic epithelium.
These ducts, however, as well as the coelomic epithelium, may assume excretory functions.
There is usually one pair of coelomic ducts leading from the pericardium to the exterior, and these are the excretory organs or kidneys, formerly known as the organs of Bojanus.
There are thus two pairs of coelomic ducts.
In animals which exhibit typical segmentation or metamerism, such as segmented worms (Chaetopoda), each segment or metamere possesses its own coelomic cavity, a pair of coelomic ducts, and a pair of nephridia.
The structure of the Mollusca in the greater number of cases agrees with the hypothesis that the primitive form was unsegmented, and therefore had but one pair of coelomic ducts and one pair of nephridia.
Among the Amphineura we find one pair of coelomic ducts in the Aplacophora, two pairs in the Chitons.
It is possible therefore to regard the latter condition as secondary, and to conclude that the separate genital ducts have been derived from the original single pair of coelomic ducts, as in Lamellibranchs.
On the whole, then, the most probable conclusion is that the original ancestral form of the Mollusca was unsegmented, possessed one pair of true nephridia, and one pair of coelomic ducts whose function was to conduct the generative products to the exterior.
The system of blood-vessels is entirely separate from the coelomic cavities.
They invariably disappear before the adult stage is reached, but their presence in the larva is evidence that the ancestral mollusc possessed a pair of true nephridia quite distinct from the coelomic excretory organs, which are so characteristic of existing forms in the adult condition.
These break off and, lying in the coelomic fluid, break up into spermatozoa.
There should also be only a single coelom, or a pair of lateral coelomic cavities.
In the male there are a right and a left protrusible penis in every genital segment, into which opens the nephridium and a sperm-sac. The wide funnels of the nephridia of this region are possibly of coelomic origin.
A, Oral tentacles (28 to 32 in full-grown animals, 20 to 24 in half-grown specimens); B, praeoral hood or praeoral epipleur; C, plicated ventral surface of atrial chamber; D l, D 17, D26, gonads, twenty-six pairs, coincident with myotomes io to 36; E, metapleur or lateral ridge on atrial epipleur; F, atripore, coincident with myotome 36; G', G ' 5, G34, double ventral fin rays, extending from myotomes 37 to 52, but having no numerical relation to them; H, position of anus, between myotomes 51 and 52; I, notochord, projecting beyond myotomes; K7, K27, K62, myotomes or muscular segments of body-wall, 62 in number; L '°°, L230, L253, dorsal fin rays, about 250 in number, the hard substance of the ray being absent at the extreme ends of the body (these have no constant numerical relation to the myomeres); M, notochord as seen through the transparent myotomes, the thin double-lined spaces being the connective-tissue septa and the broader spaces the muscular tissue of the myotomes; N, position of brown funnel of left side (atrio-coelomic canal); 0, nerve tube resting on notochord.
(3) Portion of (2) enlarged to show atrio-coelomic canals ("brown funnels" of Lankester).
Section through region of atrio-coelomic canals, v.
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).
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.