2012, ISBN: 9781130661231

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustra… Meer...

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1909 Excerpt: ...when in exposed positions, may have a tactile function. The pallial sensory organs and the ospharadium, which occur near the gills, are probably olfactory organs to test the condition of the water. The optic tentacles of pulmonates and the dorsal tentacles of opisthobranchs are stated by some authorities to be olfactory organs. Ciliated ridges in the mantle furrows of certain chitons are supposed to have an olfactory function. Auditory organs are absent in the Amphineura alone among mollusks. The auditory organ consists of a closed, fluid-filled vesicle, or 'otocyst,' in which a varying number of otoliths are inclosed. Sensory cells occur on the wall of the otocyst, which bear sensory hairs. The otocysts of cephalopods have been shown to have a statical function also. The simplest visual organs of mollusks are optic pits. The retina lies at the base of the pit, and along this base the optic nerve spreads out. The retina consists of cells with and without pigment. External to the retina is a gelatinous cuticle or even a lens. By the approach and fusion of the edges of the pit the vesicular eye is formed. That part of the epithelium which is free from pigment constitutes the cornea. This type of eye occurs on the tentacles of gastropods. By a further development the simple cup-like eye is differentiated into the complex, highly developed organ of cephalopods. The mantle eyes of the scallop resemble vertebrate eyes in that the visual rods are turned away from the light, that is to say, they are directed toward the body. The sense organs of echinoderms comprise the auditory organs and eyes. The auditory organs are of two types. The first type is represented in holothurians, and consists of an otocyst which is lined with ciliated epithelium, container and otolit... Weight:3.16 lbs, RareBooksClub.com, 3/6/2012 0:00:00<