Page:The New International Encyclopædia 1st ed. v. 13.djvu/770

* MOLLTJSK. 692 mOLLUSK. ■with water is thrust out between the valves of the shell, so that by means of it the clam can dig deep into the mud. or the freshwater mussel can plow its way through the sand. In the snails (q.v.) the foot forms a flattened creeping disk extending along the whole length of the body. See illustrations under Coxe-siiell; Coxch,, etc. A BIVALVE MOVING. A fregh-water mussel (Unio) moving through the sand toward the left, by means ot its e.teuded 'foot.' In the clam (q.v.) and most bivalves the hinder end of the bodv is prolonged into a double siphon, popularly called the 'head,' through the lower division of which enters the water laden with microscopic animals and larvse, which pass through the mantle-cavity into the mouth, which is situated at the opposite end from the siphons. The ujijier division of the siphon opens out op- posite till' iT.il of tlie intestine, which makes two and a half turns in the central or 'visceral mass.' which is composed mostly of the ovaries. The bivalves breathe by a pair of leaf-like gills, on each side of the visceral mass between it and the mantle; in the gastropods and cephalopods the gills are plume-like processes, called 'ctenidia.' There is in mollusks a definite heart, which in the primitive forms is three-chambered, i.e. a ventricle and two auricles. The ventricle in the clam and most other bivalves surrounds the in- testine: the arteries and veins are well devel- oped: the blood is colorless. iloUusks as a rule dill'er from other animals in the nervous system, the ganglia, connected by threads (commissures), being grouped around the oesophagus, one ]);>ir (the brain) situated above: another, the ])edal ganglia, in the foot, and the pair of visceral gan- glia nearer the middle of the body, anil innervat- ing the siphon, gills, digestive canal, and heart. See Gastroi'oo.v : (■|,.M; (Ivsteu: 1)K( ai'oi). : and the accompanying illustrations and plates. Sense-organs vary much in situation, number, nnd size. The eyes of the scallop, which leads an active life, leaping out of the saiul and skip- ping over the surface, arc large, highly devel- oped, and numerous, being situated arnind the edges of the mantle; in tho.se mollusks which burrow (clam) or are fixed (oyster), or live in holes in limestone or coral, etc.. the eyes are atrophied. In the squid and otlier cephalopods they are large and as complicated as those of a fish. The eyes of the land snails are borne at the ends of the tentacles, but in burrowing ma- rine gastrojMiils they may be wanting through dis- use, though j)resent in the larvsr. Our knowledge of the nature of vision in gastro- pod mollusks has hitherto been very scanty, no observations having been made since those of Lesp^s in 18.t1. until experiments by Willem brought out the following residts: ( 1 ) Snails pos- sess a well developed power of touch, permitting them to perceive feel)le jars of the soil beneath, and slight movements of the surrounding media. (2) They see very badly and cliri«t their move- ments principally by means of the senses of smell and touch. They form a confused image of large objects at an estimated distance of about a centi- meter (.40 inch). They clearly distinguish the form of objects only at a distance of one or two millimeters. (3) The fresh-water snails do not liave distinct vision at any distance. (4) There does not exist in mollusks a special vision of movements, such as insects possess. In general pulmonate mollusks respond to the action of light, in a degree diirering in dill'erent species. They have dermatoptic perceptions, which vary much in intensity in diU'ercnt species. Organs of orientation, equilibrium, or bearing are the 'otocysts.' In bivalves a single one is situated in the centre of the foot; in cephalopods they are large and placed one behind each eye. The oto- cysl is a primitive form of ear. being a sack con- taining a minute particle of lime, the otolith. The sense of smell resides in the tentacles of the snail, the olfactory nerve branching out at the end. Also near the visceral ganglia is a group of sense-cells (osphradia) supposed to be either olfactory or for testing the purity of the water entering by the respiratory current or. in snails, passing directly into the mantle-cavity. Thus the organs of smell appear to be in all the forms represented by groups of s])ecialized cells, which arc persistent in nerve-supply and po-iition in all mollusks. showing that the sense of smell is all- important. Special organs of taste are as yet unknown in mollusks. Excretory organs are a single pair of highly modified tubes (nepliridia) situated one on each side of the body ju.-t below the heart. See Xervous System. eVolutio.x of THE; Excretory System. C'omi'ar.^tive Anat- omy OF THE; and similar titles. The female ovaries and male reproductive glands are impaired. The eggs arc small and ex- ceedingly numerous, and pass out in bivalves among the folds of the gills, where the young develop. The animal after hatching passes through a gastrula. trochosphere. and veliger stage, the last so called from the two ciliated tlaps on each side of the head. Toward the end H LARVAL BTAOE8 OF MOLLCSKS. A. A trncho.^phere of the pockle (faniiniu^: r, elllftt«Hl eriiwii ; tf, Huki'IIuui. B. Ve!l(.'er Hta«e. with the whell •]<•■ vek>pin^ : v. velum : m. mouth : //. liver lohes : /. .stoniiieh ; i. intestine; mt, mantle; /, foot; ml. musole; n. nervous ganglion. of the veliger stage the shell appears, arising from the incipient mantle as a cup-shaped body in both bivalves and univalves, but the hinge and separate valves are indicated very early in the IVIeeypoda. In the young I'nio, or fresh-water mussel, the development history is more con- densed. The velum is wanting or vestigial, and the young live between the gills of the parent