MOLLUSK. An animal of the phylum Mot lusca (q.v.). Mollusks are usually easy to dis tinguish from other animals on account of their shell, whence they are eonnulady called 'shell fish': but the more we study their• development and morphology the more difficult is it to draw a definite line between them and certain worm like forms. In their early development they travel along apparently the same de•elopmmenta1 path as the worms ( planarians as well as an nelids), and then diverge into a separate path. That the type is a very snccos.;ful one is proved by the enormous number of species both living and extinct—a success evidently due to the pro tection afforded them by their shell. In bivalves as well as univalves (Gastropoda) the shell is more or less solid, is conquised mainly of carbon ate of lime and secreted by the mantle.
A bivalve mollusk like the clain is completely protected by a pair of solid calcareous shells con nected by a hinge consisting of a large tooth (in most bivalves there are three teeth) and liga ment. The shells are eqnivalve or with both valves alike, but unlike at each end, the head end being more rounded. On the interior are two muscular impressions or 'scars' made by the two adductor muscles. The shell of gastropods is spiral, and that part of the animal contained in the skin is asymmetrical, the twist or torsion be ing due to gravity o• lopping over of the young shell in the larva stage.
As to the most primitive forms we are in the dark. The most characteristic mollusks are the cephalopods. They are bilateral and very highly modified, with a well marked In-ad, eontaining two highly specialized eyes and two ears. The concentrated ganglia form a brain with cartila ginous protections, while the parts around the mouth are modified to form the tentacles and flume]. The heart and blood vessels are highly specialized.
The 'foot' is a mollification of the part of the mantle below and behind the mouth; it varies greatly in shape, and is by disuse wanting in oysters and other fixed forms; in the pelecypods or 1,ival•es it is tongue-shaped, and by being filled 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 fresh-water mussel can plow its way through the sand. In the snails tg.v.) the foot forms a flattened creeping disk extending along the whole length of the body. :See illustrations under CoNE-SIIELL; CONCH. etc.
In the clam (q.v.) and most bivalves the hinder end of the body is prolonged into a double siphon, popularly called the 'head.' through the lower division of which enters the water laden with mieniscolde animals and which pass through the mantle-cavity into the mouth, which is situated at the opposite end from the siphons. The upper division of the siphon opens out op posite the end of the 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 'etenidia.' There is in mollusks a definite heart, which in the primitive forms is three-ehandwred, 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. .Mollusks as a rule differ from other animals in the nervous system, the ganglia, connected by threads (commissures). being grouped around the (esophagus. one pair the brain) situated above; another. the pedal ganglia. in the foot. and the pair of visceral gan glia nearer the middle of the body. and innervat ing the siphon. gills. digestive canal. and heart.
See (:AsTnoeonA ; M ; )YSTElt DEC troDA : and the aveompanying illustrations and plate,.
Sense-organs vary min+ in situation, number, and size. The eyes of the scallop, which leads an active life, leaping out of the sand and skip ping over the are large, highly devel oped, and numerous, being situated around the edges of the mantle; in those mollusks which burrow (clam) or are fixed (oyster). or live in holes in limestone or eoral. etc., eyes are atrophied. In the squid and other cephalopods limy are large and as eomplicated as those of a Tlw eyes of the land snails are borne at the ends of the tentacles. hut ill burrowing ma rine gastropods they may be wanting through dis use, though present in the Our knowledge of the nature of vision in gastro pod mollusks has hitherto been very scanty, no observations having been made since those of T.espi‘s in 1R,1, until experiments by Willem brought out the following results: Ill Snails pos se..; a wribdoveloped power touch, permitting them to perceive feeble jars of the soil beneath, and slight movements of the surrounding media. (21 They see very badly and direct their move melds principally by means of the senses of smell and touch. They form a confused image of large objects at au estimated distance of about a centi meter 1.40 inch). They clearly distinguish the form of objects only at a distance of one or two millimeters. 13) The fresh-water snails do not have 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 differing in different species. They have dermatoptie perceptions, which vary munch in intensity in different species. Organs of orientation, equilibrium, or hearing are time 'otocysts.' Tn 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 cyst is a primitive form of ear, being a sack con taining a minute particle of lime. the otolith. The sense of smell resides in tine tentacles of the snail, the olfactory nerve branching out at the end. Also near the visceral ganglia is a group of sense-eells tosphradia) 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 forin, represented by groups of specialized cells, which are persistent in nerve-supply and position 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 (nephridia) situated one on each side of the body just below the heart. See NERVoUS SYSTEM. EvommoN oe THE; EXCRETORY SYSTEM, CoMPARATIVE ANAT OMY OF THE; and similar titles.
The female ovaries and male reproductive glands are unpaired. The eggs are 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. troehospliere. and %Alger stage, the last so called from the two ciliated (laps on each side of the bead. Toward the end 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 l'eleeypoda. In the young l'nio. or freshwater mussel, the development history is more con densed. The velum is wanting or vestigial. and the young live between the gills of the parent fastened to each other by their threads (byssus). The shells (glochidium) are triangular, broader than long, with the apex hooked.