. Elementary text-book of zoology. 188 ANNUL AT A. a pair of otocysts. These are situated laterally on the peris- tomium and are supplied by nerves from the brain. They consist of spherical sacs communi- cating with the exterior by fine ducts. The cells lining the Fig. 114.—Section through the Otocyst of Arenicola. (After AsHWORTH and Gamble.) External Aperture. ^^ . , ,. ^^ sac are sensory and the cavity contains a number of loose concretions or otolitJu which appear to be sand-grains. On the prostomium is a ciliated pit called the nuchal- organ, a probable sense-organ allied t
. Elementary text-book of zoology. 188 ANNUL AT A. a pair of otocysts. These are situated laterally on the peris- tomium and are supplied by nerves from the brain. They consist of spherical sacs communi- cating with the exterior by fine ducts. The cells lining the Fig. 114.—Section through the Otocyst of Arenicola. (After AsHWORTH and Gamble.) External Aperture. ^^ . , ,. ^^ sac are sensory and the cavity contains a number of loose concretions or otolitJu which appear to be sand-grains. On the prostomium is a ciliated pit called the nuchal- organ, a probable sense-organ allied to the paired ciliated pits of Polygordius. Arenicola has no prostomial tentacles, but the prostomium is produced into two lobes which are also probably sensory. The muscular system is well developed and consists of a circular layer under the ectoderm and a longi- tudinal layer inside it. There are also diagonal fibres running from the lateral lines to the mid-ventral line. Nearly all the animals usually called "worms " move by the system of circular and longitudinal muscles. The body of the " worm " which contains coelomic fluid acts much like an elongated bladder or sausage- skin filled with fluid. When the circular muscles contract they press on the ccelomic fluid which forces out each end and hence elongates the worm, reducing its calibre. When the longitudinal muscles contract the body is shortened, the coelomic fluid at the same time forcing the walls outwards. Hence the alternate movements of the two muscular. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Masterman, Arthur Thomas. Edinburgh, E. & S. Livingstone
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Keywords: ., bookcentury1900, bookcollectionbiod, bookdecade1900, bookyear1902
