Chapter XXIII: Excretory Organs (1)

Excretory organs consist of coiled or branched and often ciliated tubes, with an excretory pore opening on the outer surface of the body, and as a rule an internal ciliated orifice placed in the body-cavity. In forms provided with a true vascular system, there is a special development of capillaries around the glandular part of the excretory organs. In many instances the glandular cells of the organs are filled with concretions of uric acid or some similar product of nitrogenous waste.

There is a very great morphological and physiological similarity between almost all the forms of excretory organ found in the animal kingdom, but although there is not a little to be said for holding all these organs to be derived from some common prototype, the attempt to establish definite homologies between them is beset with very great difficulties.

Platyelminthes. Throughout the whole of the Platyelminthes these organs are constructed on a well-defined type, and in the Rotifera excretory organs of a similar form to those of the Platyelminthes are also present.

These organs (Fraipont, No. 513) are more or less distinctly paired, and consist of a system of wide canals, often united into a network, which open on the one hand into a pair of large tubes leading to the exterior, and on the other into fine canals which terminate by ciliated openings, either in spaces between the connective-tissue cells (Platyelminthes), or in the body-cavity (Rotifera). The fine canals open directly into the larger ones, without first uniting into canals of an intermediate size.

The two large tubes open to the exterior, either by means of a median _posteriorly_ placed contractile vesicle, or by a pair of vesicles, which have a ventral and _anterior_ position. The former type is characteristic of the majority of the Trematoda, Cestoda, and Rotifera, and the latter of the Nemertea and some Trematoda. In the Turbellaria the position of the external openings of the system is variable, and in a few Cestoda (Wagner) there are lateral openings on each of the successive proglottides, in addition to the terminal openings. The mode of development of these organs is unfortunately not known.

Mollusca. In the Mollusca there are usually present two independent pairs of excretory organs--one found in a certain number of forms during early larval life only[247], and the other always present in the adult.

[247] I leave out of consideration an external renal organ found
in many marine Gasteropod larvæ, _vide_ Vol. II. p. 280.

The larval excretory organ has been found in the pulmonate Gasteropoda (Gegenbaur, Fol[248], Rabl), in Teredo (Hatschek), and possibly also in Paludina. It is placed in the anterior region of the body, and opens ventrally on each side, a short way behind the velum. It is purely a larval organ, disappearing before the close of the veliger stage. In the aquatic Pulmonata, where it is best developed, it consists on each side of a V-shaped tube, with a dorsally-placed apex, containing an enlargement of the lumen. There is a ciliated cephalic limb, lined by cells with concretions, and terminating by an internal opening near the eye, and a non-ciliated pedal limb opening to the exterior[249].

[248] H. Fol, "Etudes sur le dével. d. Mollusques." Mem. III.
_Archiv d. Zool. expér. et génér._, Vol. VIII.

[249] The careful observations of Fol seem to me nearly
conclusive in favour of this limb having an external opening, and
the statement to the reverse effect on p. 280 of Vol. II. of this
treatise, made on the authority of Rabl and Bütschli, must
probably be corrected.

Two irreconcilable views are held as to the development of this system. Rabl (Vol. ii. No. 268) and Hatschek hold that it is developed in the mesoblast; and Rabl states that in Planorbis it is formed from the anterior mesoblast cells of the mesoblastic bands. A special mesoblast cell on each side elongates into two processes, the commencing limbs of the future organ. A lumen is developed in this cell, which is continued into each limb, while the continuations of the two limbs are formed by perforated mesoblast cells.

According to Fol these organs originate in aquatic Pulmonata as a pair of invaginations of the epiblast, slightly behind the mouth. Each invagination grows in a dorsal direction, and after a time suddenly bends on itself, and grows ventralwards and forwards. It thus acquires its V-shaped form.

In the terrestrial Pulmonata the provisional excretory organs are, according to Fol, formed as epiblastic invaginations, in the same way as those in the aquatic Pulmonata, but have the form of simple non-ciliated sacks, without internal openings.

The permanent renal organ of the Mollusca consists typically of a pair of tubes, although in the majority of the Gasteropoda one of the two tubes is not developed. It is placed considerably behind the provisional renal organ.

Each tube, in its most typical form, opens by a ciliated funnel into the pericardial cavity, and has its external opening at the side of the foot. The pericardial funnel leads into a glandular section of the organ, the lining cells of which are filled with concretions. This section is followed by a ciliated section, from which a narrow duct leads to the exterior.

As to the development of this organ the same divergence of opinion exists as in the case of the provisional renal organ.

Rabl's careful observations on Planorbis (Vol. II. No. 268) tend to shew that it is developed from a mass of mesoblast cells, near the end of the intestine. The mass becomes hollow, and, attaching itself to the epiblast on the left side of the anus, acquires an opening to the exterior. Its internal opening is not established till after the formation of the heart. Fol gives an equally precise account, but states that the first rudiment of the organ arises as a solid mass of epiblast cells. Lankester finds that this organ is developed as a paired invagination of the epiblast in Pisidium, and Bobretzky also derives it from the epiblast in marine Prosobranchiata. In Cephalopoda on the other hand Bobretzky's observations (I conclude this from his figures) indicate that the excretory sacks of the renal organs are derived from the mesoblast.

Polyzoa. Simple excretory organs, consisting of a pair of ciliated canals, opening between the mouth and the anus, have been found by Hatschek and Joliet in the Entoproctous Polyzoa, and are developed, according to Hatschek, by whom they were first found in the larva, from the mesoblast.

Brachiopoda. One or rarely two (Rhynchonella) pairs of canals, with both peritoneal and external openings, are found in the Brachiopoda. They undoubtedly serve as genital ducts, but from their structure are clearly of the same nature as the excretory organs of the Chætopoda described below. Their development has not been worked out.

Chætopoda. Two forms of excretory organ have been met with in the Chætopoda. The one form is universally or nearly universally present in the adult, and typically consists of a pair of coiled tubes repeated in every segment. Each tube has an internal opening, placed as a rule in the segment in front of that in which the greater part of the organ and the external opening are situated.

There are great variations in the structure of these organs, which cannot be dealt with here. It may be noted however that the internal opening may be absent, and that there may be _several internal openings for each organ_ (Polynoe). In the Capitellidæ moreover several pairs of excretory tubes have been shewn by Eisig (No. 512) to be present in each of the posterior segments.

The second form of excretory organ has as yet only been found in the larva of Polygordius, and will be more conveniently dealt with in connection with the development of the excretory system of this form.

There is still considerable doubt as to the mode of formation of the excretory tubes of the Chætopoda. Kowalevsky (No. 277), from his observations on the Oligochæta, holds that they develop as outgrowths of the epithelial layer covering the posterior side of the dissepiments, and secondarily become connected with the epidermis.

Hatschek finds that in Criodrilus they arise from a continuous linear thickening of the somatic mesoblast, immediately beneath the epidermis, and dorsal to the ventral band of longitudinal muscles. They break up into S-shaped cords, the anterior end of each of which is situated in front of a dissepiment, and is formed at first of a single large cell, while the posterior part is continued into the segment behind. The cords are covered by a peritoneal lining, which still envelopes them, when in the succeeding stage they are carried into the body-cavity. They subsequently become hollow, and their hinder ends acquire openings to the exterior. The formation of their internal openings has not been followed.

Kleinenberg is inclined to believe that the excretory tubes take their origin from the epiblast, but states that he has not satisfactorily worked out their development.

The observations of Eisig (No. 512) on the Capitellidæ support Kowalevsky's view that the excretory tubes originate from the lining of the peritoneal cavity.

Hatschek (No. 514) has given a very interesting account of the development of the excretory system in Polygordius.

[FIG. 383. POLYGORDIUS LARVA. (After Hatschek.)

_m._ mouth; _sg._ supraoesophageal ganglion; _nph._ nephridion;
_me.p._ mesoblastic band; _an._ anus; _ol._ stomach.]

The excretory system begins to be formed, while the larva is still in the trochospere stage (fig. 383, _nph_), and consists of a provisional excretory organ, which is placed in front of the future segmented part of the body, and occupies a position very similar to that of the provisional excretory organ found in some Molluscan larvæ (_vide_ p. 681).

Hatschek, with some shew of reason, holds that the provisional excretory organs of Polygordius are homologous with those of the Mollusca.

In its earliest stage the provisional excretory organ of Polygordius consists of a pair of simple ciliated tubes, each with an anterior funnel-like opening situated in the midst of the mesoblast cells, and a posterior external opening. The latter is placed immediately in front of what afterwards becomes the segmented region of the embryo. While the larva is still unsegmented, a second internal opening is formed for each tube (fig. 383, _nph_) and the two openings so formed may eventually become divided into five (fig. 384 A), all communicating by a single pore with the exterior.

When the posterior region of the embryo becomes segmented, paired excretory organs are formed in each of the posterior segments, but the account of their development, as given by Hatschek, is so remarkable that I do not think it can be definitely accepted without further confirmation.

From the point of junction of the two main branches of the larval kidney there grows backwards (fig. 384 B), to the hind end of the first segment, a very delicate tube, only indicated by its ciliated lumen, its walls not being differentiated. Near the front end of this tube a funnel, leading into the larval body cavity of the head, is formed, and subsequently the posterior end of the tube acquires an external opening, and the tube distinct walls. The communication with the provisional excretory organ is then lost, and thus the excretory tube of the first segment is established.

The excretory tubes in the second and succeeding segments are formed in the same way as in the first, _i.e._ by the continuation of the lumen of the hind end of the excretory tube from the preceding segment, and the subsequent separation of this part as a separate tube.

[FIG. 384. DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE EXCRETORY
SYSTEM OF POLYGORDIUS. (After Hatschek.)]

The tube may be continued with a sinuous course through several segments without a distinct wall. The external and internal openings of the permanent excretory tubes are thus secondarily acquired. The internal openings communicate with the permanent body-cavity. The development of the permanent excretory tubes is diagrammatically represented in fig. 384 C and D.

The provisional excretory organ atrophies during larval life.

If Hatschek's account of the development of the excretory system of Polygordius is correct, it is clear that important secondary modifications must have taken place in it, because his description implies that there sprouts from the anterior excretory organ, while it has its own external opening, a posterior duct, which does not communicate either with the exterior or with the body-cavity! Such a duct could have no function. It is intelligible either (1) that the anterior excretory organ should lead into a longitudinal duct, opening posteriorly; that then a series of secondary openings into the body-cavity should attach themselves to this, that for each internal opening an external should subsequently arise, and the whole break up into separate tubes; or (2) that behind an anterior provisional excretory organ a series of secondary independent segmental tubes should be formed. But from Hatschek's account neither of these modes of evolution can be deduced.

Gephyrea. The Gephyrea may have three forms of excretory organs, two of which are found in the adult, and one, similar in position and sometimes also in structure, to the provisional excretory organ of Polygordius, has so far only been found in the larvæ of Echiurus and Bonellia.

In all the Gephyrea the so-called 'brown tubes' are apparently homologous with the segmented excretory tubes of Chætopods. Their main function appears to be the transportation of the generative products to the exterior. There is but a single highly modified tube in Bonellia, forming the oviduct and uterus; a pair of tubes in the Gephyrea inermia, and two or three pairs in most Gephyrea armata, except Bonellia. Their development has not been studied.

In the Gephyrea armata there is always present a pair of posteriorly placed excretory organs, opening in the adult into the anal extremity of the alimentary tract, and provided with numerous ciliated peritoneal funnels. These organs were stated by Spengel to arise in Bonellia as outgrowths of the gut; but in _Echiurus_ Hatschek (No. 515) finds that they are developed from the somatic mesoblast of the terminal part of the trunk. They soon become hollow, and after attaching themselves to the epiblast on each side of the anus, acquire external openings. They are not at first provided with peritoneal funnels, but these parts of the organs become developed from a ring of cells at their inner extremities; and there is at first but a single funnel for each vesicle. The mode of increase of the funnels has not been observed, nor has it been made out how the organs themselves become attached to the hindgut.

The provisional excretory organ of Echiurus is developed at an early larval stage, and is functional during the whole of larval life. It at first forms a ciliated tube on each side, placed in front of that part of the larva which becomes the trunk of the adult. It opens to the exterior by a fine pore on the ventral side, immediately in front of one of the mesoblastic bands, and appears to be formed of perforated cells. It terminates internally in a slight swelling, which represents the normal internal ciliated funnel. The primitively simple excretory organ becomes eventually highly complex by the formation of numerous branches, each ending in a slightly swollen extremity. These branches, in the later larval stages, actually form a network, and the inner end of each main branch divides into a bunch of fine tubes. The whole organ resembles in many respects the excretory organ of the Platyelminthes.

In the larva of _Bonellia_ Spengel has described a pair of provisional excretory tubes, opening near the anterior end of the body, which are probably homologous with the provisional excretory organs of Echiurus (_vide_ Vol. II., fig. 162 C, _se_).

Discophora. As in many of the types already spoken of, permanent and provisional excretory organs may be present in the Discophora. The former are usually segmentally arranged, and resemble in many respects the excretory tubes of the Chætopoda. They may either be provided with a peritoneal funnel (Nephelis, Clepsine) or have no internal opening (Hirudo).

Bourne[250] has shewn that the cells surrounding the main duct in the medicinal Leech are perforated by a very remarkable network of ductules, and the structure of these organs in the Leech is so peculiar that it is permissible to state with due reserve their homology with the excretory organs of the Chætopoda.

[250] "On the Structure of the Nephridia of the Medicinal Leech."
_Quart. J. of Micr. Science_, Vol. XX. 1880.

The excretory tubes of Clepsine are held by Whitman to be developed in the mesoblast.

There are found in the embryos of Nephelis and Hirudo certain remarkable provisional excretory organs the origin and history of which are not yet fully made out. In Nephelis they appear as one (according to Robin), or (according to Bütschli) as two successive pairs of convoluted tubes on the dorsal side of the embryo, which are stated by the latter author to develop from the scattered mesoblast cells underneath the skin. At their fullest development they extend, according to Robin, from close to the head to near the ventral sucker. Each of them is U-shaped, with the open end of the U forwards, each limb of the U being formed by two tubes united in front. No external opening has been clearly made out. Fürbringer is inclined from his own researches to believe that they open laterally. They contain a clear fluid.

In Hirudo, Leuckart has described three similar pairs of organs, the structure of which he has fully elucidated. They are situated in the posterior part of the body, and each of them commences with an enlargement, from which a convoluted tube is continued for some distance backwards; the tube then turns forwards again, and after bending again upon itself opens to the exterior. The anterior part is broken up into a kind of labyrinthic network.

The provisional excretory organs of the Leeches cannot be identified with the anterior provisional organs of Polygordius and Echiurus.

Arthropoda. Amongst the Arthropoda _Peripatus_ is the only form with excretory organs of the type of the segmental excretory organs of the Chætopoda[251].

[251] _Vide_ F. M. Balfour, "On some points in the Anatomy of
Peripatus Capensis." _Quart. J. of Micr. Science_, Vol. XIX.
1879.

These organs are placed at the bases of the feet, in the lateral divisions of the body-cavity, shut off from the main median division of the body-cavity by longitudinal septa of transverse muscles.

Each fully developed organ consists of three parts:

(1) A dilated vesicle opening externally at the base of a foot. (2) A coiled glandular tube connected with this, and subdivided again into several minor divisions. (3) A short terminal portion opening at one extremity into the coiled tube and at the other, as I believe, into the body cavity. This section becomes very conspicuous, in stained preparations, by the intensity with which the nuclei of its walls absorb the colouring matter.

In the majority of the Tracheata the excretory organs have the form of the so-called Malpighian tubes, which always (_vide_ Vol. II.) originate as a pair of outgrowths of the epiblastic proctodæum. From their mode of development they admit of comparison with the anal vesicles of the Gephyrea, though in the present state of our knowledge this comparison must be regarded as somewhat hypothetical.

The antennary and shell-glands of the Crustacea, and possibly also the so-called dorsal organ of various Crustacean larvæ appear to be excretory, and the two former have been regarded by Claus and Grobben as belonging to the same system as the segmental excretory tubes of the Chætopoda.

Nematoda. Paired excretory tubes, running for the whole length of the body in the so-called lateral line, and opening in front by a common ventral pore, are present in the Nematoda. They do not appear to communicate with the body cavity, and their development has not been studied.

Very little is known with reference either to the structure or development of excretory organs in the Echinodermata and the other Invertebrate types of which no mention has been so far made in this Chapter.

_Excretory organs and generative ducts of the Craniata._

Although it would be convenient to separate, if possible, the history of the excretory organs from that of the generative ducts, yet these parts are so closely related in the Vertebrata, in some cases the same duct having at once a generative and a urinary function, that it is not possible to do so.

The excretory organs of the Vertebrata consist of three distinct glandular bodies and of their ducts. These are (1) a small glandular body, usually with one or more ciliated funnels opening into the body cavity, near the opening of which there projects into the body cavity a vascular glomerulus. It is situated very far forwards, and is usually known as the head-kidney, though it may perhaps be more suitably called, adopting Lankester's nomenclature, the _pronephros_. Its duct, which forms the basis for the generative and urinary ducts, will be called the _segmental duct_.

(2) The Wolffian body, which may be also called the _mesonephros_. It consists of a series of, at first, segmentally (with a few exceptions) arranged glandular canals (_segmental tubes_) primitively opening at one extremity by funnel-shaped apertures into the body cavity, and at the other into the segmental duct. This duct becomes in many forms divided longitudinally into two parts, one of which then remains attached to the segmental tubes and forms the _Wolffian or mesonephric duct_, while the other is known as the _Müllerian duct_.

(3) The kidney proper or _metanephros_. This organ is only found in a completely differentiated form in the amniotic Vertebrata. Its duct is an outgrowth from the Wolffian duct.

The above parts do not coexist in full activity in any living adult member of the Vertebrata, though all of them are found together in certain embryos. They are so intimately connected that they cannot be satisfactorily dealt with separately.

Elasmobranchii. The excretory system of the Elasmobranchii is by no means the most primitive known, but at the same time it forms a convenient starting point for studying the modifications of the system in other groups. The most remarkable peculiarity it presents is the absence of a pronephros. The development of the Elasmobranch excretory system has been mainly studied by Semper and myself.

The first trace of the system makes its appearance as a knob of mesoblast, springing from the intermediate cell-mass near the level of the hind end of the heart (fig. 385 A, _pd_). This knob is the rudiment of the abdominal opening of the segmental duct, and from it there grows backwards to the level of the anus a solid column of cells, which constitutes the rudiment of the segmental duct itself (fig. 385 B, _pd_). The knob projects towards the epiblast, and the column connected with it lies between the mesoblast and epiblast. The knob and column do not long remain solid, but the former acquires an opening into the body cavity (fig. 421, _sd_) continuous with a lumen, which makes its appearance in the column (fig. 386, _sd_). The knob forms the only structure which can be regarded as a rudiment of the pronephros.

[FIG. 385. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL
CLEFTS.

The sections illustrate the development of the segmental duct (_pd_)
or primitive duct of the pronephros. In A (the anterior of the two
sections) this appears as a solid knob (_pd_) projecting towards the
epiblast. In B is seen a section of the column which has grown
backwards from the knob in A.

_spn._ rudiment of a spinal nerve; _mc._ medullary canal; _ch._
notochord; _X._ subnotochordal rod; _mp._ muscle-plate; _mp´._
specially developed portion of muscle-plate; _ao._ dorsal aorta;
_pd._ segmental duct; _so._ somatopleure; _sp._ splanchnopleure;
_pp._ body cavity; _ep._ epiblast; _al._ alimentary canal.]

While the lumen is gradually being formed, the segmental tubes of the mesonephros become established. They appear to arise as differentiations of the parts of the primitive lateral plates of mesoblast, placed between the dorsal end of the body cavity and the muscle-plate (fig. 386, _st_)[252], which are usually known as the intermediate cell-masses.

[252] In my original account of the development I held these
tubes to be invaginations of the peritoneal epithelium. Sedgwick
(No. 549) was led to doubt the accuracy of my original statement
from his investigations on the chick; and from a re-examination
of my specimens he arrived at the results stated above, and which
I am now myself inclined to adopt.

The lumen of the segmental tubes, though at first very small, soon becomes of a considerable size. It appears to be established in the position of the section of the body cavity in the intermediate cell-mass, which at first unites the part of the body cavity in the muscle-plates with the permanent body cavity. The lumen of each tube opens at its lower end into the dorsal part of the body cavity (fig. 386, _st_), and each tube curls obliquely backwards round the inner and dorsal side of the segmental duct, near which it at first ends blindly.

[FIG. 386. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY
YOUNGER THAN 28 F.

_sp.c._ spinal canal; _W._ white matter of spinal cord; _pr._
posterior nerve-roots; _ch._ notochord; _x._ subnotochordal rod;
_ao._ aorta; _mp._ muscle-plate; _mp´._ inner layer of muscle-plate
already converted into muscles; _Vr._ rudiment of vertebral body;
_st._ segmental tube; _sd._ segmental duct; _sp.v._ spiral valve;
_v._ subintestinal vein; _p.o._ primitive generative cells.]

One segmental tube makes its appearance for each somite (fig. 265), commencing with that immediately behind the abdominal opening of the segmental duct, the last tube being situated a few segments behind the anus. Soon after their formation the blind ends of the segmental tubes come in contact with, and open into the segmental duct, and each of them becomes divided into four parts. These are (1) a section carrying the peritoneal opening, known as the peritoneal funnel, (2) a dilated vesicle into which this opens, (3) a coiled tubulus proceeding from (2), and terminating in (4) a wider portion opening into the segmental duct. At the same time, or shortly before this, each segmental duct unites with and opens into one of the horns of the cloaca, and also retires from its primitive position between the epiblast and mesoblast, and assumes a position close to the epithelium lining the body cavity (fig. 380, _sd_). The general features of the excretory organs at this period are diagrammatically represented in the woodcut (fig. 387). In this fig. _pd_ is the segmental duct and _o_ its abdominal opening; _s.t_ points to the segmental tubes, the finer details of whose structure are not represented in the diagram. The mesonephros thus forms at this period an elongated gland composed of a series of isolated coiled tubes, one extremity of each of which opens into the body cavity, and the other into the segmental duct, which forms the only duct of the system, and communicates at its front end with the body cavity, and behind with the cloaca.

[FIG. 387. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN
ELASMOBRANCH EMBRYO.

_pd._ segmental duct. It opens at _o_ into the body cavity and at
its other extremity into the cloaca; _x._ line along which the
division appears which separates the segmental duct into the
Wolffian duct above and the Müllerian duct below; _s.t._ segmental
tubes. They open at one end into the body cavity, and at the other
into the segmental duct.]

The next important change concerns the segmental duct, which becomes longitudinally split into two complete ducts in the female, and one complete duct and parts of a second duct in the male. The manner in which this takes place is diagrammatically represented in fig. 387 by the clear line _x_, and in transverse section in figs. 388 and 389. The resulting ducts are (1) the Wolffian duct or mesonephric duct (_wd_), dorsally, which remains continuous with the excretory tubules of the mesonephros, and ventrally (2) the oviduct or Müllerian duct in the female, and the rudiments of this duct in the male. In the female the formation of these ducts takes place (fig. 389) by a nearly solid rod of cells being gradually split off from the ventral side of all but the foremost part of the original segmental duct. This nearly solid cord is the Müllerian duct (_od_). A very small portion of the lumen of the original segmental duct is perhaps continued into it, but in any case it very soon acquires a wide lumen (fig. 389 A). The anterior part of the segmental duct is not divided, but remains continuous with the Müllerian duct, of which its anterior pore forms the permanent peritoneal opening[253] (fig. 387). The remainder of the segmental duct (after the loss of its anterior section, and the part split off from its ventral side) forms the Wolffian duct. The process of formation of these ducts in the male differs from that in the female chiefly in the fact of the anterior undivided part of the segmental duct, which forms the front end of the Müllerian duct, being shorter, and in the column of cells with which it is continuous being from the first incomplete.

[253] Five or six segmental tubes belong to the region of the
undivided anterior part of the segmental duct, which forms the
front end of the Müllerian duct; but they appear to atrophy very
early, without acquiring a definite attachment to the segmental
duct.

[FIG. 388. DIAGRAMMATIC REPRESENTATION OF A TRANSVERSE SECTION OF A
SCYLLIUM EMBRYO ILLUSTRATING THE FORMATION OF THE WOLFFIAN AND
MÜLLERIAN DUCTS BY THE LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT.

_mc._ medullary canal; _mp._ muscle-plate; _ch._ notochord; _ao._
aorta; _cav._ cardinal vein; _st._ segmental tube. On the left side
the section passes through the opening of a segmental tube into the
body cavity. On the right this opening is represented by dotted
lines, and the opening of the segmental tube into the Wolffian duct
has been cut through; _w.d._ Wolffian duct; _m.d._ Müllerian duct.
The section is taken through the point where the segmental duct and
Wolffian duct have just become separate; _gr._ the germinal ridge
with the thickened germinal epithelium; _l._ liver; _i._ intestine
with spiral valve.]

[FIG. 389. FOUR SECTIONS THROUGH THE ANTERIOR PART OF THE SEGMENTAL
DUCT OF A FEMALE EMBRYO OF SCYLLIUM CANICULA.

The figure shews how the segmental duct becomes split into the
Wolffian or mesonephric duct above, and Müllerian duct or oviduct
below.

_wd._ Wolffian or mesonephric duct; _od._ Müllerian duct or oviduct;
_sd._ segmental duct.]

The segmental tubes of the mesonephros undergo further important changes. The vesicle at the termination of each peritoneal funnel sends a bud forwards towards the preceding tubulus, which joins the fourth section of it close to the opening into the Wolffian duct (fig. 390, _px_). The remainder of the vesicle becomes converted into a Malpighian body (_mg_). By the first of these changes a tube is established connecting each pair of segments of the mesonephros, and though this tube is in part aborted (or only represented by a fibrous band) in the anterior part of the excretory organs in the adult, and most probably in the hinder part, yet it seems almost certain that the secondary and tertiary Malpighian bodies of the majority of segments are developed from its persisting blind end. Each of these secondary and tertiary Malpighian bodies is connected with a convoluted tubulus (fig. 391, _a.mg_), which is also developed from the tube connecting each pair of segmental tubes, and therefore falls into the primary tubulus close to its junction with the segmental duct. Owing to the formation of the accessory tubuli the segments of the mesonephros acquire a compound character.

[FIG. 390. LONGITUDINAL VERTICAL SECTION THROUGH PART OF THE
MESONEPHROS OF AN EMBRYO OF SCYLLIUM.

The figure contains two examples of the budding of the vesicle of a
segmental tube (which forms a Malpighian body in its own segment) to
unite with the tubulus in the preceding segment close to its opening
into the Wolffian (mesonephric) duct.

_ge._ epithelium of body-cavity; _st._ peritoneal funnel of
segmental tube with its peritoneal opening; _mg._ Malpighian body;
_px._ bud from Malpighian body uniting with preceding segment.]

[FIG. 391. THREE SEGMENTS OF THE ANTERIOR PART OF THE MESONEPHROS OF
A NEARLY RIPE EMBRYO OF SCYLLIUM CANICULA AS A TRANSPARENT OBJECT.

The figure shews a fibrous band passing from the primary to the
secondary Malpighian bodies in two segments, which is the remains of
the outgrowth from the primary Malpighian body.

_st.o._ peritoneal funnel; _p.mg._ primary Malpighian body; _a.mg._
accessory Malpighian body; _w.d._ mesonephric (Wolffian) duct.]

The third section of each tubulus becomes by continuous growth, especially in the hinder segments, very bulky and convoluted.

The general character of a slightly developed segment of the mesonephros at its full growth may be gathered from fig. 391. It commences with (1) a peritoneal opening, somewhat oval in form (_st.o_) and leading directly into (2) a narrow tube, the segmental tube, which takes a more or less oblique course backwards, and, passing superficially to the Wolffian duct (_w.d_), opens into (3) a Malpighian body (_p.mg_) at the anterior extremity of an isolated coil of glandular tubuli. This coil forms the third section of each segment, and starts from the Malpighian body. It consists of a considerable number of rather definite convolutions, and after uniting with tubuli from one, two, or more (according to the size of the segment) accessory Malpighian bodies (_a.mg_) smaller than the one into which the segmental tube falls, eventually opens by (4) a narrowish collecting tube into the Wolffian duct at the posterior end of the segment. Each segment is probably completely isolated from the adjoining segments, _and never has more than one peritoneal funnel and one communication with the Wolffian duct_.

Up to this time there has been no distinction between the anterior and posterior tubuli of the mesonephros, which alike open into the Wolffian duct. The collecting tubes of a considerable number of the hindermost tubuli (ten or eleven in Scyllium canicula), either in some species elongate, overlap, while at the same time their openings travel backward so that they eventually open by apertures (not usually so numerous as the separate tubes), on nearly the same level, into the hindermost section of the Wolffian duct in the female, or into the urinogenital cloaca, formed by the coalesced terminal parts of the Wolffian ducts, in the male; or in other species become modified, by a peculiar process of splitting from the Wolffian duct, so as to pour their secretion into a single duct on each side, which opens in a position corresponding with the numerous ducts of the other species (fig. 392). In both cases the modified posterior kidney-segments are probably equivalent to the permanent kidney or metanephros of the amniotic Vertebrates, and for this reason the numerous collecting tubes or single collecting tube, as the case may be, will be spoken of as ureters. The anterior tubuli of the primitive excretory organ retain their early relation to the Wolffian duct, and form the permanent Wolffian body or mesonephros.

The originally separate terminal extremities of the Wolffian ducts always coalesce, and form a urinal cloaca, opening by a single aperture, situated at the extremity of the median papilla behind the anus. Some of the peritoneal openings of the segmental tubes in Scyllium, or in other cases all the openings, become obliterated.

In the male the anterior segmental tubes undergo remarkable modifications, and become connected with the testes. Branches appear to grow from the first three or four or more of them (though probably not from their peritoneal openings), which pass to the base of the testis, and there uniting into a longitudinal canal, form a network, and receive the secretion of the testicular ampullæ (fig. 393, _nt_). These ducts, the vasa efferentia, carry the semen to the Wolffian body, but before opening into the tubuli of this body they unite into a canal known as the _longitudinal canal of the Wolffian body_ (_l.c_), from which pass off ducts equal in number to the vasa efferentia, each of which normally ends in a Malpighian corpuscle. From the Malpighian corpuscles so connected there spring the convoluted tubuli, forming the generative segments of the Wolffian body, along which the semen is conveyed to the Wolffian duct (_v.d_). The Wolffian duct itself becomes much contorted and acts as vas deferens.

Figs. 392 and 393 are diagrammatic representations of the chief constituents of the adult urinogenital organs in the two sexes. In the adult female (fig. 392), there are present the following parts:

(1) The oviduct or Müllerian duct (_m.d_) split off from the segmental duct of the kidneys. Each oviduct opens at its anterior extremity into the body cavity, and behind the two oviducts have independent communications with the general cloaca.

(2) The mesonephric ducts (_w.d_), the other product of the segmental ducts of the kidneys. They end in front by becoming continuous with the tubulus of the anterior _persisting_ segment of the mesonephros on each side, and unite behind to open by a common papilla into the cloaca. The mesonephric duct receives the secretion of the anterior tubuli of the primitive mesonephros.

(3) The ureter which carries off the secretion of the kidney proper or metanephros. It is represented in my diagram in its most rare and differentiated condition as a single duct connected with the posterior segmental tubes.

(4) The segmental tubes (_s.t_) some of which retain their original openings into the body cavity, and others are without them. They are divided into two groups, an anterior forming the mesonephros or Wolffian body, which pours its secretion into the Wolffian duct; and a posterior group forming a gland which is probably equivalent to the kidney proper of amniotic Craniata, and is connected with the ureter.

[FIG. 392. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN
AN ADULT FEMALE ELASMOBRANCH.

_m.d._ Müllerian duct; _w.d._ Wolffian duct; _s.t._ segmental tubes;
five of them are represented with openings into the body cavity, the
posterior segmental tubes form the mesonephros; _ov._ ovary.]

[FIG. 393. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN
AN ADULT MALE ELASMOBRANCH.

_m.d._ rudiment of Müllerian duct; _w.d._ Wolffian duct, marked _vd_
in front and serving as vas deferens; _s.t._ segmental tubes; two of
them are represented with openings into the body cavity; _d._
ureter; _t._ testis; _nt._ canal at the base of the testis; _VE._
vasa efferentia; _lc._ longitudinal canal of the Wolffian body.]

In the male the following parts are present (fig. 393):

(1) The Müllerian duct (_m.d_), consisting of a small rudiment attached to the liver, representing the foremost end of the oviduct of the female.

(2) The mesonephric duct (_w.d_) which precisely corresponds to the mesonephric duct of the female, but, in addition to serving as the duct of the Wolffian body, also acts as a vas deferens (_vd_). In the adult male its foremost part has a very tortuous course.

(3) The ureter (_d_), which has the same fundamental constitution as in the female.

(4) The segmental tubes (_s.t_). The posterior tubes have the same arrangement in both sexes, but in the male modifications take place in connection with the anterior tubes to fit them to act as transporters of the semen.

Connected with the anterior tubes there are present (1) the vasa efferentia (_VE_), united on the one hand with (2) the central canal in the base of the testis (_nt_), and on the other with the longitudinal canal of the Wolffian body (_lc_). From the latter are seen passing off the successive tubuli of the anterior segments of the Wolffian body, in connection with which Malpighian bodies are typically present, though not represented in my diagram.

Apart from the absence of the pronephros the points which deserve notice in the Elasmobranch excretory system are (1) The splitting of the segmental duct into Wolffian (mesonephric) and Müllerian ducts. (2) The connection of the former with the mesonephros, and of the latter with the abdominal opening of the segmental duct which represents the pronephros of other types. (3) The fact that the Müllerian duct serves as oviduct, and the Wolffian duct as vas deferens. (4) The differentiation of a posterior section of the mesonephros into a special gland foreshadowing the metanephros of the Amniota.

Cyclostomata. The development of the excretory system amongst the Cyclostomata has only been studied in Petromyzon (Müller, Fürbringer, and Scott).

The first part of the system developed is the segmental duct. It appears in the embryo of about 14 days (Scott) as a solid cord of cells, differentiated from the somatic mesoblast near the dorsal end of the body cavity. This cord is at first placed immediately below the epiblast, and grows backwards by a continuous process of differentiation of fresh mesoblast cells. It soon acquires a lumen, and joins the cloacal section of the alimentary tract before the close of foetal life. Before this communication is established, the front end of the duct sends a process towards the body cavity, the blind end of which acquires a ciliated opening into the latter. A series of about four or five successively formed outgrowths from the duct, one behind the other, give rise to as many ciliated funnels opening into the body cavity, and each communicating by a more or less elongated tube with the segmental duct. These funnels, which have a metameric arrangement, constitute the pronephros, the whole of which is situated in the pericardial region of the body cavity.

On the inner side of the peritoneal openings of each pronephros there is formed a vascular glomerulus, projecting into the body cavity, and covered by peritoneal epithelium. For a considerable period the pronephros constitutes the sole functional part of the excretory system.

A mesonephros is formed (Fürbringer) relatively late in larval life, as a segmentally arranged series of solid cords, derived from the peritoneal epithelium. These cords constitute the rudiments of the segmental tubes. They are present for a considerable portion of the body cavity, extending backwards from a point shortly behind the pronephros. They soon separate from the peritoneal epithelium, become hollowed out into canals, and join the segmental duct. At their blind extremity (that originally connected with the peritoneal epithelium) a Malpighian body is formed.

The pronephros is only a provisional excretory organ, the atrophy of which commences during larval life, and is nearly completed when the Ammocoete has reached 180 mm. in length. Further changes take place in connection with the excretory system on the conversion of the Ammocoete into the adult.

The segmental ducts in the adult fall into a common urinogenital cloaca, which opens on a papilla behind the anus. This cloaca also communicates by two apertures (abdominal pores) with the body cavity. The generative products are carried into the cloaca by these pores; so that their transportation outwards is not performed by any part of the primitive urinary system. The urinogenital cloaca is formed by the separation of the portion of the primitive cloaca containing the openings of the segmental ducts from that connected with the alimentary tract.

The mesonephros of the Ammocoete undergoes at the metamorphosis complete atrophy, and is physiologically replaced by a posterior series of segmental tubes, opening into the hindermost portion of the segmental duct (Schneider).

In Myxine the excretory system consists (1) of a highly developed pronephros with a bunch of ciliated peritoneal funnels opening into the pericardial section of the body cavity. The coiled and branched tubes of which the pronephros is composed open on the ventral side of the anterior portion of the segmental duct, which in old individuals is cut off from the posterior section of the duct. On the dorsal side of the portion of the segmental duct belonging to the pronephros there are present a small number of diverticula, terminating in glomeruli: they are probably to be regarded as anterior segmental tubes. (2) Of a mesonephros, which commences a considerable distance behind the pronephros, and is formed of straight extremely simple segmental tubes opening into the segmental duct (fig. 385).

The excretory system of Myxine clearly retains the characters of the system as it exists in the larva of Petromyzon.

Teleostei. In most Teleostei the pronephros and mesonephros coexist through life, and their products are carried off by a duct, the nature of which is somewhat doubtful, but which is probably homologous with the mesonephric duct of other types.

The system commences in the embryo (Rosenberg, Oellacher, Götte, Fürbringer) with the formation of a groove-like fold of the somatic layer of peritoneal epithelium, which becomes gradually constricted into a canal; the process of constriction commencing in the middle and extending in both directions. The canal does not however close anteriorly, but remains open to the body cavity, thus giving rise to a funnel equivalent to the pronephric funnels of Petromyzon and Myxine. On the inner side of this funnel there is formed a glomerulus, projecting into the body cavity; and at the same time that this is being formed the anterior end of the canal becomes elongated and convoluted. The above structures constitute a pronephros, while the posterior part of the primitive canal forms the segmental duct.

[FIG. 394. PORTIONS OF THE MESONEPHROS OF MYXINE. (From Gegenbaur;
after J. Müller.)

_a._ segmental duct; _b._ segmental tube; _c._ glomerulus; _d._
afferent, _e._ efferent artery.

B represents a portion of A highly magnified.]

The portion of the body cavity with the glomerulus and peritoneal funnel of the pronephros (fig. 395, _po_) soon becomes completely isolated from the remainder, so as to form a closed cavity (_gl_). The development of the mesonephros does not take place till long after that of the pronephros. The segmental tubes which form it are stated by Fürbringer to arise from solid ingrowths of peritoneal epithelium, developed successively from before backwards, but Sedgwick informs me that they arise as differentiations of the mesoblastic cells _near_ the peritoneal epithelium. They soon become hollow, and unite with the segmental duct. Malpighian bodies are developed on their median portions. They grow very greatly in length, and become much convoluted, but the details of this process have not been followed out.

The foremost segmental tubes are situated close behind the pronephros, while the hindermost are in many cases developed in the postanal continuations of the body cavity. The pronephros appears to form the swollen cephalic portion of the kidney of the adult, and the mesonephros the remainder; the so-called caudal portion, where present, being derived (?) from the postanal segmental tubes.

In some cases the cephalic portion of the kidneys is absent in the adult, which probably implies the atrophy of the pronephros; in other instances the cephalic portion of the kidneys is the only part developed. Its relation to the embryonic pronephros requires however further elucidation.

[FIG. 395. SECTION THROUGH THE PRONEPHROS OF A TROUT AND ADJACENT
PARTS TEN DAYS BEFORE HATCHING.

_pr.n._ pronephros; _po._ opening of pronephros into the isolated
portion of the body cavity containing the glomerulus; _gl._
glomerulus; _ao._ aorta; _ch._ notochord; _x._ subnotochordal rod;
_al._ alimentary tract.]

In the adult the ducts in the lower part of the kidneys lie as a rule on their outer borders, and almost invariably open into a urinary bladder, which usually opens in its turn on the urinogenital papilla immediately behind the genital pore, but in a few instances there is a common urinogenital pore.

In most Osseous Fish there are true generative ducts continuous with the investment of the generative organs. It appears to me most probable, from the analogy of Lepidosteus, to be described in the next section, that these ducts are split off from the primitive segmental duct, and correspond with the Müllerian ducts of Elasmobranchii, etc.; though on this point we have at present no positive embryological evidence (_vide_ general considerations at the end of the Chapter). In the female Salmon and the male and female Eel the generative products are carried to the exterior by abdominal pores. It is possible that this may represent a primitive condition, though it is more probably a case of degeneration, as is indicated by the presence of ducts in the male Salmon and in forms nearly allied to the Salmonidæ.

The coexistence of abdominal pores and generative ducts in Mormyrus appears to me to demonstrate that the generative ducts in Teleostei cannot be derived from the coalescence of the investment of the generative organs with the abdominal pores.

Ganoidei. The true excretory gland of the adult Ganoidei resembles on the whole that of Teleostei, consisting of an elongated band on each side--the mesonephros--an anterior dilatation of which probably represents the pronephros.

There is in both sexes a Müllerian duct, provided, except in Lepidosteus, with an abdominal funnel, which is however situated relatively very far back in the abdominal cavity. _The Müllerian ducts appear to serve as generative canals in both sexes._ In Lepidosteus they are continuous with the investment of the generative glands, _and thus a relation between the generative ducts and glands, very similar to that in Teleostei, is brought about_.

[FIG. 396. SECTION THROUGH THE TRUNK OF A LEPIDOSTEUS EMBRYO ON THE
SIXTH DAY AFTER IMPREGNATION.

_mc._ medullary cord; _ms._ mesoblast; _sg._ segmental duct; _ch._
notochord; _x._ subnotochordal rod; _hy._ hypoblast.]

Posteriorly the Müllerian ducts and the ducts of the mesonephros remain united. The common duct so formed on each side is clearly the primitive segmental duct. It receives the secretion of a certain number of the posterior mesonephric tubules, and usually unites with its fellow to form a kind of bladder, opening by a single pore into the cloaca, behind the anus. The duct which receives the secretion of the anterior mesonephric tubules is the true mesonephric or Wolffian duct.

The development of the excretory system, which has been partially worked out in Acipenser and Lepidosteus[254], is on the whole very similar to that in the Teleostei. The first portion of the system to be formed is the segmental duct. In Lepidosteus this duct is formed as a groove-like invagination of the somatic peritoneal epithelium, precisely as in Teleostei, and shortly afterwards forms a duct lying between the mesoblast and the epiblast (fig. 396, _sg_). In Acipenser (Salensky) however it is formed as a solid ridge of the somatic mesoblast, as in Petromyzon and Elasmobranchii (fig. 397, _Wg_).

[254] Acipenser has been investigated by Fürbringer, Salensky,
Sedgwick, and also by myself, and Lepidosteus by W. N. Parker and
myself.

[FIG. 397. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN
ACIPENSER EMBRYO. (After Salensky.)

_Rf._ medullary groove; _Mp._ medullary plate; _Wg._ segmental duct;
_Ch._ notochord; _En._ hypoblast; _Sgp._ mesoblastic somite; _Sp._
parietal part of mesoblastic plate.]

In both forms the ducts unite behind with the cloaca, and a pronephros of the Teleostean type appears to be developed. This gland is provided with but one[255] peritoneal opening, which together with the glomerulus belonging to it becomes encapsuled in a special section of the body cavity. The opening of the pronephros of Acipenser into this cavity is shewn in fig. 398, _pr.n._ At this early stage of Acipenser (larva of 5 mm.) I could find no glomerulus.

[255] I have not fully proved this point, but have never found
more than one opening.

The mesonephros is formed some distance behind, and some time after the pronephros, both in Acipenser and Lepidosteus, so that in the larvæ of both these genera the pronephros is for a considerable period the only excretory organ. In Lepidosteus especially the development of the mesonephros occurs very late.

The development of the mesonephros has not been worked out in Lepidosteus, but in Acipenser the anterior segmental tubes become first established as (I believe) solid cords of cells, attached at one extremity to the peritoneal epithelium on each side of the insertion of the mesentery, and extending upwards and outwards round the segmental duct[256]. The posterior segmental tubes arise later than the anterior, and (as far as can be determined from the sections in my possession) they are formed independently of the peritoneal epithelium, on the dorsal side of the segmental duct.

[256] Whether the segmental tubes are formed as ingrowths of the
peritoneal epithelium, or _in situ_, could not be determined.

[FIG. 398. TRANSVERSE SECTION THROUGH THE REGION OF THE STOMACH OF A
LARVA OF ACIPENSER 5 MM. IN LENGTH.

_st._ epithelium of stomach; _yk._ yolk; _ch._ notochord, below
which is a subnotochordal rod; _pr.n._ pronephros; _ao._ aorta;
_mp._ muscle-plate formed of large cells, the outer parts of which
are differentiated into contractile fibres; _sp.c._ spinal cord;
_b.c._ body cavity.]

In later stages (larvæ of 7-10 mm.) the anterior segmental tubes gradually lose their attachment to the peritoneal epithelium. The extremity near the peritoneal epithelium forms a Malpighian body, and the other end unites with the segmental duct. At a still later stage wide peritoneal funnels are established, for at any rate a considerable number of the tubes, leading from the body cavity to the Malpighian bodies. These funnels have been noticed by Fürbringer, Salensky and myself, but their mode of development has not, so far as I know, been made out. The funnels appear to be no longer present in the adult. The development of the Müllerian ducts has not been worked out.

Dipnoi. The excretory system of the Dipnoi is only known in the adult, but though in some respects intermediate in character between that of the Ganoidei and Amphibia, it resembles that of the Ganoidei in the important feature of the Müllerian ducts serving as genital ducts in both sexes.