. Outlines of zoology. d to theTeleostean bulbus, an optic chiasma, as opposed to the decussatecondition in Teleosts, and an intestinal spiral valve which is absent inTeleosts, they do not seem to form a natural division. 572 PJSCES—FISHES. skull. The tail is sometimes quite symmetrical ordiphycercal, but in most cases it is heterocercal at first,and acquires a secondary symmetry termed homocerqal;for while the end of the notochord in the young forms isbent upwards as usual, the subsequent development of raysproduces an apparent symmetry. The scales are in mostcases relatively soft. The roof o
. Outlines of zoology. d to theTeleostean bulbus, an optic chiasma, as opposed to the decussatecondition in Teleosts, and an intestinal spiral valve which is absent inTeleosts, they do not seem to form a natural division. 572 PJSCES—FISHES. skull. The tail is sometimes quite symmetrical ordiphycercal, but in most cases it is heterocercal at first,and acquires a secondary symmetry termed homocerqal;for while the end of the notochord in the young forms isbent upwards as usual, the subsequent development of raysproduces an apparent symmetry. The scales are in mostcases relatively soft. The roof of the fore-brain is withoutnervous matter. The optic nerves are remarkable, becausethey cross one another without interlacing (decussate).The partitions between the gill-clefts disappear; so, insteadof the pouches seen in Elasmobranchs, there is, on eachside, one branchial chamber, covered over by an opercularfold. Into this chamber the comb-like gills, borne bythe branchial arches, project freely. There is usually a. Fig. 306.—The goldfish (Cyprinus auratus). rudimentary gill or pseudobranch associated with the is no spiracle. In most, a swim-bladder is developedfrom the dorsal side of the gullet. The duct of the swim-bladder may remain open (Physostomous), as in herring,salmon, and carp; or it may be closed (Physoclystous), as inperch and cod. There is no spiral valve in the intestine,and the food canal ends in front of, and separate from, thegenital and urinary apertures or aperture. The base ofthe ventral aorta is swollen into a non-contractile bulbusarteriosus, but there is no conus, unless very exceptionally,as in Butirinus. A remarkable peculiarity is that thegonads are usually continuous with their ducts. The ovaare numerous, usually small and fertilised in the segmentation is meroblastic, and there is usually adistinct larval stage. DIPNOI. 573 The Teleosts include the great majority of living fishes, whichare classified in thirteen sub-orders
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
