. The Cambridge natural history. Zoology. STRUCTURE TENTACLES, ETC. 45 branchial siphon into the branchial sac. At the base of the siphon, just about the line of junction of the ectoderm of the stoniodaeuni with the endoderm of the mesenteron, is placed a circle of simple hair-like tentacles (Fig. 18, tn) which stand out at right angles to the wall, and more or less completely meet in the centre to form a delicate, sensory grid or sieve through which all the water entering the body has to pass. These tentacles not only act mechanically, but are also sensitive although only scattered sensory ce
. The Cambridge natural history. Zoology. STRUCTURE TENTACLES, ETC. 45 branchial siphon into the branchial sac. At the base of the siphon, just about the line of junction of the ectoderm of the stoniodaeuni with the endoderm of the mesenteron, is placed a circle of simple hair-like tentacles (Fig. 18, tn) which stand out at right angles to the wall, and more or less completely meet in the centre to form a delicate, sensory grid or sieve through which all the water entering the body has to pass. These tentacles not only act mechanically, but are also sensitive although only scattered sensory cells, and no specially differentiated sense-organs are found upon them. Behind the tentacles lies the plain, or papil- ^9' ren. At. Dorsal. l-'.y- "m. Fig. 19.—Semi-diagrammatic transverse section of Ascidia, passing through the atrial aperture, seen from anterior surface, left side uppermost. At, Atrial aperture ; , atrial lobe ; , branchial sac ; d, cloaca ; con, connective ; , dorsal blood- sinus : , dorsal lamina ; end, endostyle ; , genital ducts ; i, /', intestine ; , interstigmatic vessel ; m, mantle ; , muscle-bundles ; ov, ovary ; ^, peri- branchial cavity; r, rectum ; ren, renal vesicles ; sg, stigmata ; sph, atrial sphincter ; t, test ; tr, transverse vessel ; ty, typhlosole ; , ventral blood-sinus. lated, prebranchial zone (Fig. 21, ;r.^;), bounded beliind by a pair of parallel and closely placed ciliated ridges with a groove between—the peripharyngeal bands—which encircle the anterior end of the branchial sac. The branchial sac is very large—-much the largest organ of the body—and extends almost to the posterior end of the body, while the rest of the alimentary canal lies upon its left side. The food particles, consisting of microscopic plants and animals, are carried in through the branchial aperture by the current of water, but most of them do not pass out through the gill-slits to the atrium, being entan
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
