. Elementary text-book of zoology. Zoology. SENSE ORGANS. VASCULAR SYSTEM. 307 Br tactile organs. The latter are joined to nervous expansions and special integumentary appendages (tactile hairs), and are present even in the parasitic Worms as papilla? of the outer skin connected with nerves. In the free-living worms, these tactile organs fre- quently take the form of filiform, tentacle-like appendages on the head and segments (cirri). Auditory organs are not so generally present, and are represented by auditory vesicles (otocysts) either lying on the brain (some Turbellaria and Nemertinea), or
. Elementary text-book of zoology. Zoology. SENSE ORGANS. VASCULAR SYSTEM. 307 Br tactile organs. The latter are joined to nervous expansions and special integumentary appendages (tactile hairs), and are present even in the parasitic Worms as papilla? of the outer skin connected with nerves. In the free-living worms, these tactile organs fre- quently take the form of filiform, tentacle-like appendages on the head and segments (cirri). Auditory organs are not so generally present, and are represented by auditory vesicles (otocysts) either lying on the brain (some Turbellaria and Nemertinea), or 011 the cesophageal ring (certain branchiate Worms among the Annelida}. The organs of sight are simple pigment spots in connection with nerves (eye-spots), and may be provided with refractive bodies. The ciliated pits of the Nemertinea have been regarded as organs of smell. The cup-shaped organs of the Hirudinea and Gephyrea are also sense organs. A blood vascular si/stem is wanting in the Nemathelminthes, the Rotifera, and the Platyhelminthes with the exception of the Nzmertinea. In these cases, the nu- tritive fluid passes endosmotically into the body parenchyma or into the body cavi- ty, and penetrates the tissues as a clear chyle, sometimes containing cellular elements. In the Nemertinea a blood vascular system is present, as also in the Gephyrea and Annelida. In the latter it obtains the highest development, and may have the form of a completely closed vascular system provided with pulsating trunks. In most cases a dorsal contractile longitudinal trunk and a ventral vessel can be distinguished; the two being connected in each segment by arched transverse vessels, which aie sometimes pulsatile. Where a vascular system is present, the blood does not always appear clear and colourless like the fluids of the body cavity, but sometimes has a yellow, greenish, or more frequent!)' red colour, which is in some cases connected with the presence of blood corpuscles. The function of
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
