Feeding Mechanism PDF

# Feeding Mechanism Chemosynthetic bacteria and chlorophyll-containing green plants synthesize food substances from inorganic compounds present in the environment (autotrophic). Animals cannot synthesize their own food but depend on already manufactured food present in plants and animals (heterotro...

# Feeding Mechanism Chemosynthetic bacteria and chlorophyll-containing green plants synthesize food substances from inorganic compounds present in the environment (autotrophic). Animals cannot synthesize their own food but depend on already manufactured food present in plants and animals (heterotrophic). Since the source of food is relatively scarce, procurement of food has become the main pre-occupation of animals. The form in which the food is ingested is very variable. Food may be solid or liquid. Solid food may be minute (microscopic) or bulky (macroscopic). Hence the feeding habits of animals are classified as: * Macroscopic feeding * Microscopic feeding * Fluid feeding The feeding habits vary from animal to animal depending on their systematic position: phylum, class, order, family, genera, and species. ## Microphagous Feeders Microphagous feeders ingest small food particles either suspended in water or deposited in the bottom sediments. In order to obtain the suspended food, a current of water is first created and directed into a sieve that will strain off the minute suspended food particles, which after collection, are sorted out so that only the correct sizes and type pass into the mouth. This filter or suspended feeding. Such animals make use of: * Cilia, as in ciliates, sponges, bivalve molluscs, protochordates, tadpoles. * Pseudopodia as in Amoeba and radiolarians * Mucus traps as in prosobranch gastropods, tunicates and Chaetopterus * Setae as in small crustaceans, herrings, baleen whales, African flamingos and marine broad-billed prions. Microphagous feeders which collect deposited food materials are called indirect deposit feeders. Examples include Terebella, a sedentary polychaetes. ## Macrophagous Feeders Macrophagous feeders ingest relatively large solid food substances and are of three types: * Direct Deposit feeders such as earthworms, lungworms and acorn worm ingest mud or sand or earth * Carnivores seize and capture living animals. * Herbivores have chewing, biting, rasping or grazing mouthparts for dealing with plant material. ## Fluid Feeders Fluid feeders take in plant and animal fluids such as cell sap, nectar, honey, blood, coelomic fluid, egg yolk, and albumin and dissolved food substances. Fluid feeders include those animals that absorb dissolved food through their body surfaces such as protozoan parasites and tapeworms and those that have sucking mouthparts. The latter include aphids, bees, butterflies and birds that suck plant sap, nectar or honey, and leeches, ticks, mosquitoes, tse-tse flies and vampire bats that suck blood and animal body fluids. ## Filter Feeders * **Bivalve Molluscs** e.g. Mutela, Aspataria, Egeria. The specialized structures for filter feeding are the ctenidia and labial palps. When the lateral cilia present on gill filaments beat a water current containing suspended food particles is drawn into the mantle cavity. The beat of the latero-frontal cilia directs the current into the gills surface and water passes through ostia between the filaments. The particles move along the surface of the gill to food grooves located either at the ventral or dorsal margins. The cilia in the food grooves move the minute particles that fail on them to the labial palps where further sorting out of particles takes place before being transported into the mouth. * **Amphioxus**. The pharynx and ciliated gill bars are the structures used for food collection. The pharynx is an enormously large tube whose walls are perforated by about 200 oblique vertical slits separated by gills bars. There are cilia on the sides and inner surfaces of the gill bars, the lateral cilia in them beat and draw in water through the slits into an atrium and out of the body through the atriopore. The food particles become entangled on the endostyle in the floor of the pharynx and from here cilia move the particles dorsally into an epipharynx and from this posteriorly into the midgut. * **Tadpoles of frogs or toads**: At a certain size, tadpoles of frogs and toads are entirely macrophagous. The longitudinal section of the pharynx of the tadpole of Bufo bufo. The pharynx is separated from the gill opening by septa and the openings are guarded by Lattice-like filter. Mucus secreted by the vela or septa traps the food particles and form a food string which is then swallowed. * **Other vertebrates** * Herring and Mackerel possess long thin gill-rakers which prevent the escape of copepods and other zooplankton. Similar gill rakers for feeding on plankton are found in basking-shark, Cetorhinus. * In the whole-bone whales, filtering apparatus consists of hundreds of close-set transverse baleen plates. Euphasid shrimps are strained off by this filtering device during feeding. In many birds such as the broad-billed prion, Pachyptila, freshwater ducks and the African flamingo Phoenicopterus, the upper part of the bill bears two rows of comb-like lamellae for filtering plankton. The larger P. antiquorum have coarse filters for feeding on chironomid larvae and other small invertebrates at the bottom of muds in lakes, ponds and streams. ## Indirect Deposit Feeders The sedentary polychaete worm, Terebella lives in tubes in the substratum of the sea-bed. It bears a crown of hollow tentacles which are ciliated on one surface. The tentacles are displayed in the sea water as shown in figure 1.8. During feeding, the ciliated surface of a tentacles is placed over deposited detritus. The tip of a tentacle in contact with food is flattened and the edges are pulled inwards in order to collect some of the detritus. The tentacle is then turned in a supine position so that the ciliated surface is upper. Transversely running muscles now contract to form the surface into a food groove in which the food is then moved towards and into the mouth by ciliary action and by peristaltic wave. ## Direct Deposit Feeders In the earthworms, Libyodrillus and Lumbricus, the lugworm Arenicola and the acornworm Balandlossus, there is a spindle shaped proboscis with which the soil is easily penetrated. Within this structure are the radial muscles in the pharyngeal wall whose action enable soil particles to be sucked into the mouth. Pharyngeal glands secrete mucus, for mixing ingested soil particles. There is a large thin-walled crop which acts as a storage chamber (Fig, 1.9). The muscular gizzard, lined with cuticle grinds the food particles into a pulpy mass before this is passed onto the long intestine, the principal site for digestion and absorption. Only about 1% of the ingested material is digested, the remainder is removed as a cast which is seen at the entrance the burrow. ## Herbivores There are only groups of animals that have successfully adopted herbivorous feeding habit. These are gastropod molluscs, insects and herbivorous mammals. The gastropods have radula (Fig. 1.10a) for grazing vegetation. The radula is an abraiding tooth ribbon, wide in some gastropods but narrow with stronger teeth in chitons, limpets and periwinkles for scrapping encrusted algae from hard substances. The radula contains numerous transverse and longitudinal rows of chisel-like teeth for cutting plants-parts (Fig. 1.10b). In the African giant land snáil. Archachatina and Achatina, there are about 200 longitudinal rows with about 100 teeth per transverse row in the radula. As the gastropod grazes on plan material the radula is constantly worn away anteriorly and replaced from a long-coiled part posteriorly. In the limpet, Patella, the radula ribbon is almost twice the length of the whole body. Marine gastropods feed on algae on rock surfaces. Brown algae contain algin (a polymer of mannose units), laminarin and fucoidan while the red algae contain agar (a polymer of galactose) and iridophycin. Terrestrial snails and slugs feed on leaves and succulent fruits. Insects that are strictly herbivorous include the dragon flies, grasshoppers, crickets, roaches, mantids, termites and ants. In these the mandibles and maxillae (Fig. 1.10c) have strongly toothed and heavily chitinized edges bearing pointed cusps for cutting and flat cusps for crushing vegetable matter. The gizzards or proventiculus in these insects is a very powerful and muscular triturating chamber for further crushing ingested plant material. Herbivorous mammals include the elephants (Proboscidae), horses, zebras, rhinoceroses (Perissodactyls). Goats, sheep, pigs, deer, cattle (artiodactyls) and rodents and lagomorphs. In the cow, there are no upper incisor teeth; the position is replaced by a horny pad which act as a template for cutting grasses and any other vegetable matter. Canines are absent, and there is a long space or diastema before the premolars and molar or cheek teeth. These have flat surfaces with cutting edges or cusps for thorough chewing and grinding to break the cellulose, cell walls of plants. In rodents and lagomorphs (the latter include rabbits and hares), the incisors rabbits are very large, curved and chisel-like for gnawing food. There is a pair of incisors in rodents, two in lagomorphs. The remaining incisors, canine and some anterior premolars are missing thus leaving a large diastema in front of the cheek teeth which have flat grinding surfaces with sharp transverse cusps. ## Carnivores Carnivores capture and ingest other animals; scavengers eat already dead animals and do not normally pursue and capture prey. A carnivorous habit has the advantage that the food is compact and nutritively rich in all food substances. Many carnivores ingest over-large prey and hence feeding is intermittent. In general, a carnivore must posses the means to capture and seize its prey; this may require it to pursue the prey. Swallowing prey whole requires distensible or wide mouth and foregut. Vertebrate carnivores employ modified strong, sharp and pointed teeth and claws for prehension and other suitably modified teeth for crushing and dismembering prey. Some carnivores secrete powerful proteolytic enzymes for external digestion and liquefaction of apprehended prey. Carnivores exist in all animal phyla (Read up mouth parts of coelenterates (Cnidarians), prey capture by small invertebrate animals, such as annelids. Annelids and crustaceans with the aid of nematocysts which are of various types. Penetrants contain a neurotoxin which paralyses prey when injected into its body. The thread of volvents, when discharged, winds into a tight coil around an appendage of the prey while the thread of glutinants remains straight but has adhesive properties. All three types of nematocytes are borne in hundreds in a tentacle of a Cnidarian. Tentacles that have captured a prey bend towards the hypostome and push the prey through the mouth into the enteron, a primitive gut. Gland cells in the endoderm secrete powerful enzymes which break prey into minute fragments before being ingested for intracellular digestion. Polymorphic cnidarians such as Portuguese-man-of-war, Physalia capture prey using tentacles or dactylozoids. The prey is then covered all over by hundreds of feeding polyps or individuals or gastrozoids, which pour out their powerful proteolytic enzymes over the prey, usually a fish or shellfish. The proboscis worms (Phylum Nemertea), and errant polychaetes possess rapidly eversible proboscis or introvert bearing a pair of powerful jaws. When shot out by the pressure of fluid in the sheath, the prey is seized by the pair of strong jaws. In the carnivorous polychaetes such as Nereis, Nephthys, Glycera, Eunice, Aphrodita. The introvert is equipped with paragnaths or dentricles for crushing the prey when the introvert is withdrawn. In the nemertean, the ejected proboscis entwines around the prey and immobilizes it with mucus. Starfish, e.g. Asterias, Astropecten, and other asteroids feed mainly on bivalve molluscs. These echinoderms force apart the shell valves of their prey by means of tube feet or podia. Prolonged application of suction power by the tube feet forces the adductor muscles of the bivalve to gradually relax due to fatigue. The starfish then extrudes its stomach and inserts into the mantle cavity of its prey. From the stomach and pyloric caeca are secreted powerful proteolytic enzymes which rapidly dissolves all the organs of the bivalve leaving only an empty shell, the soluble material having been into the caeca by ciliary currents. Crayfish, crabs and lobsters are benthic and feed on an indiscriminate range of slow-moving prey, mainly worms, small crustaceans, molluscs and small fish which are seized on and killed by chelipeds. They also feed on dead tissues. The mouthparts include a complex array of appendages or maxillipeds which push the food into the mouth where the heavily chitinized mandibles cut and chew it. Whelks and cones, among the prosobranch gastropods, prey on other molluscs and fish. Welks such as Natica, Murex, Thais and Buccinum have eversible proboscis which contain a radula at its tip. The sharp radula teeth are used for drilling holes into shells of other molluscs. The proboscis penetrates and the teeth cut and tear off the flesh of the prey. Cone shells (Conus), possess teeth shaped like arrows which can be fired singly at passing fish. The teeth are hollow and when shot at a prey, a powerful neurotoxin from a poison gland is injected through the tooth into the prey. Cone shells are noted for the sudden capture and swallowing of moving fish. Some gastropods swallow whole prey. Philine crushes bivalve shell with the aid of its heavily toothed gizzard. The slug Testacella swallow earthworms by means of its massive sharp-toothed odontophore and Gymnodoris, an opisthobranch, engulfs other nudibranchs by means of its distensible buccal mass. Cephalopods capture prey with their long tentacular arms which bear suckers. The squid, Loligo captures small fish, the cuttle-fish, Sepia feeds on shrimps while the octopus seizes crabs. The prey is held against the mouth by the suckered arms. A neurotoxin from the salivary glands may be injected to paralyze a struggling prey. The food is cut into pieces by the horny parrot-like beak and the radula teeth are used for drawing the pieces into the pharynx.