EXCRETORY SYSTEM. 347 EXECUTION. The excretory organs (if vertebrates are much more complicated than any we have so far consid- ered. In its most highly developed form the ver- tebrate excretory system consists of three sets of organs. The first set to arise, botli in phyloge- netie and ontogenetic development, are known as the promephos or 'head kidneys.' Usually thej arise in a more anterior position than the other kidneys. Jn position as well as in several other respects they correspond to the segmental tubules of amphioxus. They are segmentally arranged like Ihnse of amphioxus and annulates, but they are much fewer in numbers. They arise in the mesoderm of the anterior end of the co lomic wall and each tubule opens into the eoelom by a ciliated funnel. They differ from the tubules of annulates and amphioxus, however, in that each tubule does not open directly to the exterior, but pours its secretion into a common duel — the seg- mental duct — which in turn discharges into the cloaca. It has been suggested that the segmental tubules formerly poured their contents into a longitudinal groove situated on the exterior. By the sinking of the groove beneath the surface a tube was formed. The embryonic development of this duct gives little light, as to its origin. In -nine cases it is formed by a growth backward from the pronephros. In others the mesoblast or even the hypoblast seems to take an active part in its formation. The head kidney is said to be the functional excretory organ in the fish Teiras- fer and some of the other bony fishes. In Myxine and Bdellostoma it persists throughout adult life, although evidently in a somewhat degenerate con- dition; while in all the higher vertebrates ex- cept turtles and crocodiles it is rudimentary even in embryonic life. The head kidney then is present at some stage in the development of all vertebrates, although its appearance may be very fleeting in some forms, and it may be so rudimentary in others as never to he functional. When the head kidney is func- tional the mesonephros or 'middle kidnej - ' appears later in larval development than in the cases in which the pronephros is only rudimentarily de- veloped. The mesonephros consists of another series of tubules, developed from the mesoblast and usually in a position posterior to that of the head kidneys. These tubules likewise become con- nected with the segmental duct. Below the Amniota this, the Wolffian body, is the permanent kidney. Upon the appearance of the mesonephros the nephridia of the head kidneys lose their con- nection with the segmental duct. The tubules of the mesonephros also open into the body-cavity by ciliated funnels. The funnels are not always pres- ent. They are in intimate relation with a glo- merulus from the aorta. In selachians, Paul Meyer and Ruckert have observed vessels which connect the dorsal aorta with the subintestinal veins. Such a blood-supply corresponds more nearly with that afforded by the segmental or- gans of the amphioxus. In many forms the segmental duct seems to divide, or at least two ducts appear side by side. One retains its connections with the kidney tubules and is known as the "Wolffian duct. The other, the Miillerian duct, opens into the body- cavity on the one hand and as the oviduct serves to convey the eggs to the exterior. In the male the Wolffian duct becomes the genital duct. In the higher vertebrates still a third or permanent kidney arises posterior to the mesonephros, and Vol. VII. — 2a. is known as the metanephros. This is the 'kid- ney' of the higher vertebrate rhi permanent kidney consists of coiled tubules, bul a nephro stome is never present. Capillaries, formiri meruli, are also present. The duet, the ureter of the permanent kidneys of higher vertebrates (from reptiles to man), is formed from a diver- ticulum which grows forward from the posterior end of the Wolffian duct and connects with the posterior nephridia. The urimin bladder of fishes is formed by a diverticulum of the ureter. In higher vertebrates the bladder, while it has the same function, has an entirely different origin from that of fishes, for it arises as a pouch on the ventral wall of the cloaca. BlBLIOGEAPHY. I'.alfour, Cum para-tive Embry- oJotiy i London, ls.sl ) ; Sedgwick, Quarterly Jour- nal Microscopical Science, ols. w. and xxi. I I on don, 1877-78); Wiedesheim, Comparative inat- mil!/ of Vertebrates (2d ed. by Parker, London, l!- : '.i7) ; Lang, Text-Book of Comparative Anatomy i London, 1891 96) ; Quain, Lehrbuch der tno tomie des M enschen (Leipzig, 1892). EXCURSION, The. A didactic and descrip- tive poem in blank verse, by Wordsworth, orig- inally intended as a part, of The Recluse, never completed. It appeared in 1SI4. and aroused a
iolont controversy between the critics of the
old and the new' order. EXE, eks. A river rising in Exmoor, in West Somersetshire, England (Map: England, C 5). It Hows 19 miles southeast to the borders of Devonshire, and then 35 miles south through that county, past Tiverton and Exeter, into the Eng- lish Channel at Exmouth. EXECUTION (Lat. executio, performance, from exequi, exsequi, to carry out, from i < out + sequi, to follow : connected with Gk. (ireodai, hepesthai, Lith. sekti, Skt. sac, Av. hac, to fol- low. Goth, saikioan, Icel. sja, OIK!, sehan, Ger. sehen, AS. scon, Eng. see). The enforcement, by the duly constituted authorities, of a final judg- ment, order, or decree of a court of justice. In strictness, the term has reference only to the enforcement of the process of the common-law. courts, whether in civil or in criminal eases, and is not applicable to the procedure for carrying into effect the decrees of the equity courts. This is due to the radical difference in the jurisdiction exercised by the two classes of tribunals. A judgment in a court of law is merely an adjudi- cation of the rights of the contending parties, declaring that one is or is not entitled to spei ifii property or to a sum of money. In itself con- sidered it has no further force, and is ineffectual until enforced by distinct process, and this is furnished by the writ of execution, addressed to the administrative officers of the county, and by the proceedings taken thereunder. A court of equity, on the other hand, acts in personam : that is. its decree is directed to the person against whom the proceeding is brought and hinds him directly without further process. The machinery of the court is sufficient to enforce its decrees without calling upon the administrative officers of the county or the State to carry them into effect. However, in those cases in which the decree in equity directs the delivery of specific property or the pavment of a sum of money, the process of execution is coming to be employed for carrving it into effect.
