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GENERAL FEATURES and LIFE HISTORY of NEREIS (Clam Worm) (Neanthes)

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GENERAL FEATURES and LIFE HISTORY of NEREIS (Clam Worm) (Neanthes) Mob.no.--7004774289 B.Sc-Part-1 Group-A GENERAL FEATURES AND LIFE HISTORY OF NEREIS (CLAM WORM) ( NEANTHES) DR.SUJIT KUMAR DEPARTMENT OF ZOOLOGY ALLMA IQBAL COLLEGE BIHARSHARIF,NALANDA INTRODUCTION Nereis is a typical polychaete genus, living in burrows in sand or mud, often with clams, a reason for which its species are commonly known as clamworms or sandworms. Neanthes is one of the largest and most common typical marine annelid with most of the characteristics of the phylum. Nereis includes several species, of which more common are N.virens, N.cultrifera, N.dumerilli, N diversicolor, N pelagica and N succinea. The following account largely pertains to Nereis virens. Systematic Position Phylum: Annelida Class: Polychaeta Subclass: Errantea Family: Nereidae Genus: Neanthes or Nereis Species: Virens Neanthes is found on the sea-shore in the shallow water in rock crevices, hidden under the stone or sea weeds. Some live in tubular U-shaped burrows lined mucus in sand or mud at tide level. It is carnivorous and nocturnal. At night it keeps its head protruded out of the burrow in search of prey, which is usually a small crustacean, mollusc or annelid. When breeding period approaches it leaves the burrow and comes at the water surface to lend a pelagic life, and then it is called heteronereis. It is cosmopolitan in distribution found abundantly in Europe, North Pacific, America, Alaska, and other places. 1. Shape, size and colour- The body is long, narrow, slender, bilaterally symmetrical, tapering posteriorly and relatively broad anteriorly. It is slightly flattened dorsoventrally, dorsal surface being convex, while ventral surface flat or even somewhat concave. It may range from 30-40 cm in length and 2 to 6 mm in width. Different species are differently coloured. Colour may vary with age and sexual maturity. 2. Segmentation – Body of Nereis is metamerically divided into a number of metameres or segments arranged in a linear series. All the metameres are nearly alike except the last one which is rounded. Number of metameres is fairly constant for a species; about 80 in N.cultrifera and N.dumerilli and about 200 in N.virens. 3. Division of body- Body of Neanthes is divisible into three well marked regions; head, trunk and pygidium. (a) Head- lies at the anterior end and consists of two main parts- Prostomium and Periosteum. Prostomium is an anterior narrow, nearly triangular fleshy outgrowth, situated mid-dorsally in front of the mouth. Peristomium is a large ring-like structure carrying ventrally the transverse mouth. Sensory appendages and organs of head - Various sensory appendages and organs found on the head of Neanthes are: Adaptations in Tubicolous forms: Tubicolous polychaetes are those which live inside the tube in mud or sand, they are permanent tube dwellers. Eg. Chaetopterus Terebella, Serpula, Sabella, Glycera and Amphitrite. Tubicolous annelids are capable of secreting adhesive cement like substance by which, sand and mud particles etc. can be bound together for constructing tube. In Amphitrite, Sabella and Terebella, trunk segments have mucous glands which lubricate the tube. (1). Feeding mechanism: Since, the animal live permanently within tubes, they cannot move about to hunt for their prey, so food catching becomes a problem for tubicolous forms. The adaptations are such that the food may be brought to the animal within the tube. The animals develop such structures which may create water current towards the tube. The water current with it carries micro-organisms inside the burrow, the tentacles and then move towards the mouth down in the ciliated groove. All the tubicolous annelids have such cilia, whose beating maintains constant water current through the tube, so that the micro-organisms brought along the water current can be ingested by the annelid. In forms like Terebella, Serpula and Sabella, the head has many appendages, each of which is provided with a longitudinal ciliated groove whose cilia beat to produce the water current. The feeding mechanism of Chaetopterus is especially very interesting. The parapodia setae become peg like or hook like for anchoring the animal to the wall of the tube. Isabella, the gills are covered with sticky juice in which the minute food particles get attached. In Myxicola, the gill filaments are joined by a thin sheet and form a semicircle at the apex, in which food is captured. (2). Respiration: Since the animals are confined to the tube, the appendages of the head become modified to form palp and gill filaments which aid in breathing. (3). Reduction in Locomotion: Because of the absence of locomotion, locomotory organs, such as parapodia are lost. (4). Reduction of Sense Organs: Sense organs are also comparatively reduced. (5) Prey Catching Device: Since the parapodia are reduced, they cannot bring about locomotion effectively during the time when the burrowing annelid crawl freely on the sea bed. So eversible pharynx is provided with papillae and it helps in caterpillar like movement of the annelids. The eversible pharynx is an adaptation, which has a double function i.e., catching the prey and helping in locomotion. Adaptations in Burrowing Form : The burrowing polychaetes lead a peculiar life where sometimes they crawl on the seabed and sometimes burrow in the soil. So they possess some characters specialized for the burrowing mode of life and some characters, which help in crawling. For example, Arenicola, Glycera, Amphitrite, Aphrodite. (1) Body shape: For burrowing, the animals have smooth external surface and also external appendages has to be minimized so that there may be minimum friction during burrowing, thus the burrowing annelids have slender vermiform body. In Aphrodite, the body is short and bread with arched dorsal side and a flat ventral side which is annulated to form a creeping sole. (2). Reduction of Locomotion and Sensory Organs: Parapodia are lost or reduced and they have very few setae. In Arenicola, posterior end is without parapodia. The head tentacles and the palps are also reduced. Thus, the members of group Polychaeta have radiated into many forms adapted to swimming, burrowing and crawling mode of life. b) Trunk: Posterior to the head, the rest of the body which is metamerically segmented (having 80-120 segments) is called trunk. The segments are known as metameres or somites which bear a pair of parapodia. (c)Pygidium: The last segment of the body called variously as tail, anal segment or pyridium, is elongated, swollen and bears a terminal anus. It bears no parapodia but has a pair of elongated anal cirri and several minute sensory papillae. 4.Parapodia - The Parapodia are primarily the organs of locomotion used both in creeping and in swimming. They are highly muscular, well vascularised and glandular structure. Structure of the Parapodium: The segments excepting the first and last segment either lateral side fleshly, flat and hollow parapodium. Largest parapodia are encountered in the middle segments of the body, then the size of the parapodia decreases towards the two ends. Each parapodium is biramous in nature. It consists of a basal part and two distal parts. The two distal parts are- (1) dorsally placed notopodium and (2) ventrally placed neuropodium. Both these parts are subdivided into two leaf-like lobes or ligulae, a dorsal superior ligula and a ventral inferior lingula. The dorsal and ventral sides of the parapodium bear small, tentacle-like, cylindrical appendages, called dorsal and ventral cirrus respectively. Each part is supported internally by a chitinous rod, known as aciculum. Each part also bears a bundle of long, fine, stiff, chitinous bristles, the setae or chaetae, which project beyond its margin. Seta: The setae are stiff, needle-like chitinous rods which remain in bundle within a sac in the skin. The sac is known as setigerous sac. The entire bundle may be moved in various directions with the help of muscles. Each seta consists of a basal shaft with which articulates a terminal blade . Three types of setae are seen – In one type the shaft In the second type In the third type the is oar shaped. shaft is slender and shaft is comparatively the blade in narrow, thicker and the blade straight and is small and curved. elongated. 5. Nephridiopores: In each segment on the ventral side of the body near the bases of parapodia are found laterally a pair of minute openings, the excretory opening or nephridiopores by which excretory materials are removed. Body wall The body wall is divisible into (a) cuticle, (b) epidermis, (c) muscle layers and (d) parietal epithelium. a) Cuticle: It is thin, slightly brittle and chitinous having a network of fine lines on the external surface which renders an ‘iridescent lustre’. Number of minute openings are present on the cuticle through which the epidermal glands open to the exterior. b) Epidermis: This is formed by a single layer of tall columnar supporting cells and some scattered glandular and sensory cells. It is thicker on the ventral side specially near the parapodia joints. The dorsal side of the epidermis is richly supplied with blood vessels. c) Muscle layers: The outer muscular layer is circularly arranged and the inner layer runs longitudinally. The longitudinal muscles are present in four bundles - two are dorso-lateral and the remaining two are ventro-lateral. The circular muscles of dorsal and ventral sides are interconnected by two strands of oblique muscles. d) Parietal epithelium \ Peritoneum: t is a part of the coelomic epithelium which lines the outer wall of the coelom or body cavity. It is made up of a single layer of cells. Functions of body wall. Body wall of Nereis performs a variety of functions. (i) Cuticle protects the body against desiccation and mechanical injuries. (ii) Epidermis, being highly vascular, serves for respiration (iii) Epidermal glands secrete mucus for lining the burrow so as to prevent it from collapsing.
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