Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 TUNIC ATA. 87 body,—the muscles, nervous system, the digestive apparatus, the generative and circulatory organs,—lie embedded in a kind of body cavity. The nervous system is confined to a simple ganglion, the position of which near the inhalent aperture marks the dorsal surface. The nerves which radiate from the ganglion branch and pass, some to the muscles and viscera, some to the sense organs—such as eyes, auditory and tactile organs—which are found principally in the free- swimming Tu
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 TUNIC ATA. 87 body,—the muscles, nervous system, the digestive apparatus, the generative and circulatory organs,—lie embedded in a kind of body cavity. The nervous system is confined to a simple ganglion, the position of which near the inhalent aperture marks the dorsal surface. The nerves which radiate from the ganglion branch and pass, some to the muscles and viscera, some to the sense organs—such as eyes, auditory and tactile organs—which are found principally in the free- swimming Tunicates. The muscular system is chiefly developed around the respiratory cavity, and serves for the dilatation and contraction of this space as well as for closing the inhalent and exhalent pores. In the Ascidians there are three layers of muscles, an external and internal longi- Kl Nu. End M(l FIG. 559.—Salpa democratica from the side, somewhat diagrammatic. 0, Mouth ; Ph, pharyngeal cavity; Kl, cloaca: A, exhalent opening; Br, gills; N, nervous centre; Ma, mantle; M, muscular rings; Z, languet; Wb, ciliated arc; End, endostyle; Wr, ciliated groove Nu, nucleus ; C, heart. tudinal and an internal circular layer, while iu the Salps there are band-like rings of muscles embedded in the substance of the body- wall, and effecting not only the renewal of the water used in respiration, but also the movements of the free-swimming barrel- shaped body. A special organ of locomotion is present in the small Appendicularia and the free-swimming Ascidian larvse; it is placed on the ventral surface as indicated by the position of the heart, and consists of a vibratile whip-like caudal appendage supported by a notochordal rod (urochord). The alimentary canal begins in all cases with a wide pharyngeal cavity, which functions as a respiratory organ. The anterior mantle opening, which must be looked upon as the mouth, leads into this cavity. The cesophageal opening is placed at a dis
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