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256
[ANATOMY
ICHTHYOLOGY


organs of the vertebrate the archinephros develops from before backwards. The sequence is, however, not regular. A small number of tubules at the head end of the series become specially enlarged and are able to meet the excretory needs during larval existence (Pronephros): the immediately succeeding tubules remain undeveloped, and then come the tubules of the rest of the series which form the functional kidney of the adult (Mcsoneplzros).

The kidney tubules subserve the excretory function in two different ways. The wall of the tubule, bathed in blood from the posterior cardinal vein, serves to extract nitrogenous products of excretion from the blood and pass them into the lumen of the tubule. The open ciliated funnel or nephrostome at the coelomic end of the tubule serves for the passage outwards of coelomic fluid to flush the cavity of the tubule. The secretory activity of the coelomic lining is specially concentrated in certain limited areas in the neighbourhood of the nephrostomes, each such area ensheathin g a rounded mass depending into the coelom and formed of a blood-vessel coiled into a kind of skein-a glomerulus. In the case of the pronephros the glomeruli are as a rule fused together into a single glomus. In the mesonephros they remain separate and in this case the portion of coelom surrounding the glomerulus tends to be nipped off from the general coelom-to form a Malpighian body. The separation may be incomplete-the Malpighian coelom remaining in connexion with the general coelom by a narrow peritoneal canal. The splanchnocoelic end of this is usually ciliated and is termed a peritoneal funnel: it is frequently confused with the nephrostome.

Mesonephros.-The kidney of the adult fish is usually a compact gland extending over a considerable distance in an anteroposterior direction and lying immediately dorsal to the coelomic cavity. Peritoneal funnels are present in the adult of certain Selachians (e.g. Aoanlhias, Squatina), though apparently in at least some of these forms they no longer communicate with the Malpighian bodies or tubules. The kidneys of the two sides become fused together posteriorly in Protoplerus and in some Teleosts. The mesonephric ducts undergo fusion posteriorly in many cases to form a median urinary or urinogenital sinus. In the Selachians this median sinus is prolonged forwards into a pair of horn-like continuations-the sperm sacs. In Dipnoans the sinus becomes greatly dilated and forms a large, rounded, dorsally placed cloacal caecum. In Actinopterygians a urinary bladder of similar morphological import is commonly present. Gonads.-The portion of coelomic lining which gives rise to the reproductive cells retains its primitive relations most nearly in the female, where, as a rule, the genital cells are still shed into the splanchnocoele. Only in Teleostomes (Lepidosteus and most Teleosts) the modification occurs that the ovary is shut off from the splanchnocoele as a closed cavity continuous with its duct.

In a few Teleosts (Solmonidae, Mnroenidae, Cobitis) the ovary is not a closed sac, its eggs being shed into the coelom as in other groups.

The appearance of the ovary naturally varies greatly with the character of the eggs.

The portion of coelomic lining which gives rise to the male genital cells (testis) is in nearly, if not quite, all cases, shut off from the splanchnocoele. The testes are commonly elongated in form. In Dipneustil (Lepidosiren and Protopterns) the hinder portion of the elongated testis has lost its sperm-producing function, though the spermatozoa produced in the anterior portion have to traverse it in order to reach the kidney. In Pa/ypterns2 the testis is continued backwards as a “testis ridge, ” which appears to correspond with the posterior vesicular region of the testis in Lepidosiren and Protoplerns. Here also the spermatozoa pass back through the cavities of the testis ridge to reach the kidney duct. In the young Teleost*' the rudment of the duct forms a backward continuation of the 1 ]. Graham Kerr, Proc. Zool. Soc. Lond. (1901), p. 484. 2 ]. S. Budgett, Trans. Zool. Soc. Lond. xv. (1901), vol. p: 324. “ H. F. jungersen, Arb. zool. soot. Inst. Würzburg, Band xx., 1889. testis containing a network of cavities and opening as a rule posteriorly into the kidney duct. It is difficult to avoid the conclusion tha-t the testis duct of the Teleost is for the most part the equivalent morphologically of the posterior vesicular region of the testis of Polypterus and the Dipneusti. Relations of Renal and Reproductive Organs. (1) Female.-In the Selachians and Dipnoans the oviduct is of the type (Müllerian duct) present in the higher vertebrates and apparently representing a split-off portion of the archinephric duct. At its anterior end is a wide funnel»like coelomic opening. Its walls are glandular and secrete accessory coverings for the eggs. In the great majority of Teleosts and in Lepidostons the oviduct possesses no coelomic funnel, its walls being in structural continuity with the wall of the ovary. In most of the more primitive Teleostomes (Crossopterygians, sturgeons, Amie) the ovidhct has at its front end an open coelomic funnel, and it is difficult to and adequate reason for refusing to regard such oviducts as true Müllerian ducts. On this interpretation the condition characteristic of Teleosts would be due to the lips of the oviduct becoming fused with the ovarian wall, and the duct itself would be a Müllerian duct as elsewhere. -A departure from the normal arrangement is found in those Teleosts which shed their eggs into the splanchnocoele, e.g. amongst Solmonidac, the smelt

(Osrnerus) and capelin

(M allotus) possess a pair

of oviducts resembling

Mtillerian ducts while

the salmon possesses

merely a pair of genital

pores opening together

behind the anus. It

seems most probable that

the latter condition has

been derived from the

former by reduction of

the Müllerian ducts,

though it has been

argued that the converse

process has taken

place. Thegenital pores

mentioned must not be

confused with the abdominal

pores, which in

many adult fishes, particularlyinthosewlthout open peritoneal funnels,

leadfromcoelomdirectly

to the exterior in the

region of the cloacal

opening. These appear

to be recent developments,

and to have

nothing to do morphologicallywiththegenito- urinary system!

(2) M ale.-It seems

that primitively the

male reproductive elements

like the female

were shed into the

coelom and passed

thence through the

nephridialtubules. In

correlation probably

with the greatly reduced

size of these

elements they are commonly

no longer shed

mn!

772n2

/XA

/Qs

/x/Qs

is *

ig

5

mf.

=

L aim f

l'g . ' 'i'l

515; A < ali 5

I- ""~ 3

gg; 'MS

From Arch zool expérimmt

ala, by permission of

Schleicher Frenes.

FIG. 11.-Urino-Genital Organs of the right side in a male Scyllium. (After Borcea.)

m.n. 1, Anterior (genital) portion of mesonephros with its coiled duct.

rn.n. 2, Posterior (renal) portion of mesonephros. s.s, Sperm sac.

T, 'I'estis.

u, “Ureter" formed by fusion of

collecting tubes of renal portion of mesonephros.

u.g.s, Urino-genital sinus;

into the splanchnocoele, v.s, Vesicula seminal is. but are conveyed from

the testis through covered-in canals to the Malpighian bodies or kidney tubules. The system of covered-in canals forms the testicular network, the individual canals being termed vasa efferentia. In all probability the series of vasa etferentia was originally spread over the whole length of the elongated testis (cf. Lepidosteus), but in existing ishes the series is as a rule

4 E. ]. Bles, Proc. Roy. Soc. 62, 1897, p. 232.