. Outlines of zoology. rt protective. The mucus chieflyconsists of strange spiral threads which uncoil when ejectedfrom the sacs. The zigzag muscle segments or myomeres are rasping teeth are worked by a powerful muscularstructure, sometimes called a tongue. A sectionof this shows a strong muscular cylinder surrounding acartilage. 5i8 CYCLOSTOMATA. The skeleton.—The skeleton is wholly notochord persists unsegmented within a firm sheath,the skull is a simple unroofed trough, jaws are notdistinctly developed, there is only a hint of the complicatedbasket-work which
. Outlines of zoology. rt protective. The mucus chieflyconsists of strange spiral threads which uncoil when ejectedfrom the sacs. The zigzag muscle segments or myomeres are rasping teeth are worked by a powerful muscularstructure, sometimes called a tongue. A sectionof this shows a strong muscular cylinder surrounding acartilage. 5i8 CYCLOSTOMATA. The skeleton.—The skeleton is wholly notochord persists unsegmented within a firm sheath,the skull is a simple unroofed trough, jaws are notdistinctly developed, there is only a hint of the complicatedbasket-work which supports the gill-pouches of the lamprey;but the tongue, the barbules, etc., are supported by cartila-ginous rods. The tail is protocercal. Nervous system.—The brain has the usual parts, butthe cerebrum and cerebellum are little more than rudiment-ary; It is much compressed, with practical obliterationof the ventricles. The fore-brain seems to agree with thatof Ganoids and Teleosteans in having a non-nervous NTT Fig. 273.—Median longitudinal section of anterior region ofMyxine.—After Retzius and Parker. Earbule ; iV., nasal aperture ; NT.^ nasal tube with rings of cartilage ;AC, nasal capsule; BR.^ brain ; iC, spinal cord ; N.^ notochord ; G.,gut; T., cartilage of tongue; ^f., muscle ; JV77., posterior part ofnasalsac; TV., a tooth plate; jl/7., median toothonroof of mouth (;1f.)- The spinal cord is somewhat flattened, and is sheltered simplyby fibrous tissue. Throughout at least a portion of the cordthere are two dorsal roots for each ventral root. The unionof dorsal and ventral roots is only partial, and there is nosympathetic system. There is no lateral line system. The eye is without lens, cornea, iris, or muscles, and ishidden beneath the skin; the optic nerves do not crossuntil they enter the brain; the ear has only one semi-circular canal. The single nasal sac (with faired folds ofolfactory epithelium in Bdellostoma, an American relative)opens dorsall
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
