The name "Nephridium" was originally given by Sir E.
Goodrich, endorsed by Lankester, led to the opinion that under the general morphological conception of "nephridium" were included two distinct sets of organs, viz.
Bergh (for Lumbricus and Criodrilus), whose figures show a derivation of the entire nephridium from mesoblast, and an absence of any connexion between successive nephridia by any continuous band, epiblastic or mesoblastic. A midway position is taken up by Wilson, who asserts the mesoblastic formation of the funnel, but also asserts the presence of a continuous band of epiblast from which certainly the terminal vesicle of the nephridium, and doubtfully the glandular part of the tube is derived.
Vezhdovsky's figures of Rhynchelmis agree with those of Bergh in showing the backward growth of the nephridium from the funnel cell.
There are thus substantial reasons for believing that the nephridium grows backwards from a funnel as does the coelomoduct.
Usually these A, Diagram of the nephridium of Nereis diversicolor.
B, Diagram of the nephridium of Alciope, into which opens the large genital funnel (coelo mostome).
C, Small portion of the nephridium of Glycera siphonostoma, showing the canal cut through, and the solenocytes on the outer surface.
Essentially, a nephridium is a tube, generally very long and much folded upon itself, composed of a string of cells placed end to end in which the continuous lumen is excavated.
Externally, the nephridium opens by a straight part of the tube, which is often very wide, and here the intracellular lumen becomes intercellular.
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.
2) there are many of these flame cells to a single nephridium which are specialized in form, and have been termed "solenocytes" (Goodrich).
The nephridium of Nephthys scolopendroides.
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.
The Polychaeta, however, present us with another form of nephridium seen, for example, in Arenicola, where a large funnel leads into a short and wide excretory tube whose lumen is intercellular.
In the case of many Oligochaeta where there is no vascular network surrounding the nephridium, this function must be the chief one of those glands, the more elaborate process of excretion taking place in the case of nephridia surrounded by a rich plexus of blood capillaries.
In Lumbricus the connexion is a little closer; the funnel of the nephridium, in the segments in which the funnels of the gonad ducts are to be developed, persists and is continuous with the gonad duct funnels on their first appearance.
This example is similar to cases among the Polychaeta where a true nephridium is provided with a large funnel, a coelomostome, according to the nomenclature of Lankester.
In Lanice conchilega the posterior series of nephridia are connected by a thick longitudinal duct, which seems to be seen in its most reduced form in Owenia, where a duct on each side runs in the epidermis, being in parts a groove, and receives one short tubular nephridium only and occupies only one segment.
On the other hand, in most Oligochaeta the first segment has in the adult no nephridium, and in the case of Octochaetus the existence of a "head kidney" antedating the subsequently developed nephridia of the first and other segments has neither been seen nor proved to be absent.
It is further noticeable that in Rhynchelmis the covering of vesicular cells which clothes the drainpipe cells of the adult nephridium is cut off from the nephridial cells themselves and is not a peritoneal layer surrounding the nephridium.
A growth both of the funnel, which becomes multicellular, and of the rest of the nephridium produces the adult nephridia of the genera mentioned.
Thus, in Octochaetus multiporus a large nephridium opens anteriorly into the buccal cavity, and numerous nephridia in the same worm evacuate their contents into the rectum.
2; in addition, cn, nerve cord; in, intestine; nf, parts of nephridium; on, external opening of nephridium; ov, ova; 1, testis.
Ln, rn, Primarily left nephridium W, Wooden arc representing and primarily right nephthe base-line of the wall ridium.
- The same specimen viewed from the left front, so as to show the subanal tract (ff) of the larger nephridium, by which it communicates with the pericardium.
Pericardium, and is therefore a typical o, Olfactory ganglion, nephridium, was not known.
H, Papilla of the smaller nephridium, which is only represented by dotted outlines.
I, The sub-anal tract of the large nephridium given off near its papilla and seen through the unshaded smaller nephridium.
R, Nephridium (kidney).
Near this and less advanced into the branchial chamber is the single renal organ or nephridium r with its opening to the exterior r'.
- Nephridium from the o.s, External opening of segmental organ.
S.o.t, Third portion of nephridium broken off at p.f from the internal vesicle, which is not shown.
The praeoral somite develops the rudiment of a nephridium, but eventually entirely disappears.
Each nephridium is provided with either one or two funnels which open into the postseptal division of the coelom (ne.f).
- anp, Anterior neural pore; be, rudiment of buccal skeleton; c, cilia; cb, ciliated band; cc, ciliated groove; cm, cilia at margin of mouth; gl, external opening of club-shaped gland; Hn, Hatschek's nephridium; lm, left metapleur; n, notochord; pp, praeoral pit; ps, primary gill-slits, I, 5, and 13; rm, right metapleur showing through.
It, "Head kidney," with second nephridium just below it.
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.
I, C and D) is a typical but very specialized form of trochophore, provided with a branching nephridium bearing solenocytes.
Eg, Stem of the nephridium leading to no, its external aperture.
Fine caeca of the nephridium, which are seen ramifying transversely over the whole inner surface of the pedal muscular mass.
X, Aperture of the left organ of Bojanus (nephridium) exposed by cutting the attachment of the inner lamella of the inner gillplate.