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 ductus Cuvieri. It is this right ductus Cuvieri which forms the dorsal part of what is usually termed the sinus venosus. The inferior jugular veins which return the blood from the ventral side of the head also become replaced in the adult by a median unpaired vein which opens posteriorly into the sinus venosus by what probably represents the hinder end of the original right inferior jugular. It is interesting to note that in Polypterus, one of the Crossopterygian ganoids, there is a somewhat similar asymmetrical condition of inferior jugulars and ductus Cuvieri.

Oviposition of Lamprey (6).—The lamprey chooses as spawning ground a part of the stream with fairly rapid current and where the bottom is composed of sand with scattered stones. By means of the suctorial mouth, stones are removed from more or less circular area so as to form a shallow excavation. The male and female frequently work together at the task of preparing the nest. When oviposition is about to take place, the male may be seen to suddenly attach himself to the dorsal surface of the head of the female which holds on to one of the stones at the upper margin of the nest. The urogenital opening of the male, with its specially prominent papilla, is approximated to that of the female, and with a peculiar quivering movement the eggs and sperms are emitted synchronously amidst clouds of sand stirred up by the movements of the tail. The eggs fertilized thus at the moment of exit are very sticky from their coating of albumen, and become weighted down by adherent grains of sand.

Development.—The development of the lamprey is of much morphological importance from the archaic nature of the creature and from the fact that the egg is comparatively small (about 1 mm. in diameter), so that development is not greatly modified by a large mass of yolk. It has been worked out so far only in the river lamprey (7). Segmentation is complete and unequal. It, as well as the process of gastrulation, agrees in its main features with the same phenomenon in Amia, Dipnoans and Urodele amphibians. The blastopore persists as the anal opening of the adult. The mesoderm arises in a manner closely comparable with that which occurs in Amphioxus, the chief difference being that the mesoderm segments are solid instead of hollow, except in the anterior head region, where they are true hollow enterocoelic pouches. The rudiment of the central nervous system has the form of a solid keel-like ingrowth of ectoderm along the mid-dorsal line, which only secondarily becomes hollowed out—just as happens in Teleostean fishes. The young lamprey, after completing its embryonic development, passes three or four years, in fact its whole life up to the time of sexual maturity, in a prolonged larval condition in which its structure shows important differences from that of the adult. This larval stage of the fresh-water lamprey of Europe was long supposed to be a separate genus of Cyclostomes and was called Ammocoetes. The Ammocoetes lives in the mud and breathes and feeds by means of a current of water produced by ciliary action, which carries Flagellates and other microscopic organisms in through the mouth opening. Correlated with this mode of feeding the buccal cavity is without the teeth so characteristic of the adult. A number of complicated branched sensory processes grow into and nearly occlude the cavity, forming a kind of sieve with only narrow chinks through which the ingoing current passes. The water passes out by the gill openings, which in Ammocoetes open direct from pharynx to exterior. Certain arrangements of the pharyngeal wall of Ammocoetes show a remarkable resemblance to what is found in Amphioxus. The thyroid, which in the adult is a complicated ductless gland, has in the young Ammocoetes the form of a longitudinal groove of the ventral wall of the pharynx. This groove is lined by columnar cells, some carrying cilia, others being glandular and secreting sticky slime. These gland cells are arranged in four longitudinal bands. The thyroid is, in fact, in this stage in a condition corresponding exactly with the endostyle of Amphioxus. The agreement extends to function the secretion, forming sticky threads which entangle food particles. Anteriorly a pair of peripharyngeal bands pass dorsalwards, one on each side, to bend back suprapharyngeal bands which are continued to the hinder end of the pharynx. Here again the resemblance to what occurs in Amphioxus is very close.

The Ammocoetes possesses a functional liver with bileduct, while in the adult river lamprey the alimentary canal is degenerate. It has no arch elements on its notochord. Its eyes are sunk beneath the surface and nonfunctional, and they retain to a great extent an embryonic character (8). There is a rapid process of metamorphosis from the larval to the adult condition, the details of which are by no means sufficiently known. After the metamorphosis the now mature lamprey accomplishes the act of reproduction and then apparently dies almost immediately. The development of the Myxinoids is much less well known than that of the lampreys. As regards the common hagfish (Myxine glutinosa), we are indeed still in complete ignorance in regard to its developmental history in spite of persistent efforts to obtain embryological material. It seems probable that during the breeding period the hagfishes retire into some particularly inaccessible habitat. Within the last few years, however, abundant material illustrating the developmental history of Bdellostoma (9) has been obtained on the Californian coast, and this when fully worked out will give us a good idea of the general lines of Myxinoid development. The egg differs greatly from that of the lampreys. It is—as is that of Myxine—of large size, richly yolked and of a shortened-up sausage shape. It measures about 22 mm. by 8 mm. Surrounding the egg is a protective capsule of a yellow horny appearance. At one end a cap-like portion of this forms a detachable operculum, in the middle of which is a minute opening, the micropyle. Each end of the capsule is prolonged into a group of stiff processes with anchor-like expansions at their tips. Segmentation is, as in other richly yolked eggs, incomplete, confined to the germinal disk at the opercular pole. The central nervous system in Bdellostoma develops by the overarching of medullary folds, not out of a solid keel as is the case with the lampreys.

History in Time.—The softness of the skeletal tissues and the absence of scales in Cyclostomata provide little opportunity for the preservation of fossil remains of this group, and no known fossils can be referred with certainty to the Cyclostomata. The Devonian Palaeospondylus gunni has been regarded as a Cyclostome by some authors, but this relationship is at the least doubtful. Other authors have associated the Ostracoderms, the oldest known vertebrates, with this group.

—1. D. S. Jordan and B. W. Evermann, Fishes of North and Middle America (Washington, 1896), part i. p. 8; 2. L. Plate, ''SB. Ges. Naturf.'' (Berlin, Jg. 1897), p. 137; 3. F. Studnicka in Oppel’s Lehrbuch der vergleichenden mikroskopischen Anatomie der Wirbeltiere (Jena, 1905), Teil v. s. i.; 4. E. Warren, ''Q. J. Micr. Sci.'' xlv. (1902) p. 631; 5. L. Vialleton, ''Arch. d’anat. micr.'' T. vi. (1903) p. 283; 6. H. A. Surface in D. S. Jordan’s Fishes (1905), vol. i. p. 494; 7. A. E. Shipley, ''Q. J. Micr. Sci.'' xxvii. (1887), W. B. Scott, ''Journ. Morphol. i. (1887), C. Kupffer, Arch. mikr. Anat.'' xxxv. (1890), A. Goette, ''Entwick. des Flussneunauges'' (Hamburg and Leipzig, 1890); 8. C. Kohl, in Bibliotheca zoologica, Heft 13 (Cassel, 1892); 9. Bashford Dean in Kupffer’s Festschrift (Jena, 1899).

CYCLOSTYLE (Gr. , a circle, and  , a column), a term used in architecture. A structure composed of a circular range of columns without a core is cyclostylar; with a core the range would be peristyle. This is the species of edifice called by Vitruvius monopteral.

 CYGNUS (“The Swan”), in astronomy, a constellation of the northern hemisphere, mentioned by Eudoxus (4th century ) and Aratus (3rd century ), and fabled by the Greeks to be the swan in the form of which Zeus seduced Leda. Ptolemy catalogued 19 stars, Tycho Brahe 18, and Hevelius 47. In this constellation Cygni is a fine coloured double star, consisting of a yellow star, magnitude 3, and a blue star, magnitude 5. The fine double star, Cygni, separated by Sir William Herschel in 1779, has magnitudes 4 and 5; it has a companion, of magnitude 7, which, however, does not form part of the system. A double star, 61 Cygni, of magnitudes 5·3 and 5·9, was the first star whose distance was determined; its parallax is 0″·39, and it is therefore the nearest star in the northern hemisphere with the exception of Centauri. A regular variable, Cygni, has extreme magnitudes of 5 to 13·5, and its period is 406 days. Nova Cygni is a “new” star discovered by Johann Schmidt in 1876. There is also an extended nebula in the constellation.

 CYLINDER (Gr. , from  , to roll). A cylindrical surface, or briefly a cylinder, is the surface traced out by a line, named the generatrix, which moves parallel to itself and always passes through the circumference of a curve, named the directrix; the name cylinder is also given to the solid contained between such a surface and two parallel planes which intersect a generatrix. A “right cylinder” is the solid traced out by a rectangle which revolves about one of its sides, or the curved surface of this solid; the surface may also be defined as the locus of a line which passes through the circumference of a circle, and is always perpendicular to the plane of the circle. If the moving line be not perpendicular to the plane of the circle, but moves parallel to itself, and always passes through the circumference, it traces an “oblique cylinder.” The “axis” of a circular cylinder is the line joining the centres of two circular sections; it is the line through the centre of the directrix parallel to the generators. The characteristic property of all cylindrical surfaces is that the tangent planes are parallel to the axis. They are “developable” surfaces, i.e. they can be applied to a plane surface without crinkling or tearing (see ).

Any section of a cylinder which contains the axis is termed a “principal section”; in the case of the solids this section is a rectangle; in the case of the surfaces, two parallel straight lines. A section of the right cylinder parallel to the base is obviously a circle; any other section, excepting those limited by two 