Molluscs PDF

Phylum: Molluscs (Mollusca)  triploblastic animals, that have coelom – filled with hemolymph  body – soft, non-segmented  parts of body: head, muscular foot (by some animals is reduced or modified), visceral mass  have a mantle, which is a fold of the outer skin lining...

Phylum: Molluscs (Mollusca)  triploblastic animals, that have coelom – filled with hemolymph  body – soft, non-segmented  parts of body: head, muscular foot (by some animals is reduced or modified), visceral mass  have a mantle, which is a fold of the outer skin lining the shell, the mantle secretes a calcium carbonate external shell  there is a mantle cavity (palium) between mantle and body  RS – gills (aquatic species) or pulmonary sacs (terrestrial species)  IS – epidermis with mucous glands  ES – metanephridium  CS – open (only cephalopods have closed), heart has 1 ventricle and 1 or more atria (the number of atria correspond to the number of gills), hemolyph contains haemoglobin or haemocyanin,  NS – gangliated, ladder-like, have pairs of ganglia – are specialized (e.g. pedal ganglia control foot, visceral ganglia control inner organs,...)  SS – on head: 2 pairs of tentacles (hmatadlá), eyes (can be flat and very simple, but some molluscs have highly developed eyes – chamber eyes similar to eyes of vertebrates)  MS – smooth muscles of muscular foot, foot can be modified (e.g. by cephalopods – into tentacles)  DS – have complete digestive tract (starts by mouth, ends with anus – leads to mantle cavity), have radula (not bivalves) – works as a rasper and is made of chitin, have paired liver or hepatopancreas  RS – gonochorists or hermaphrodites, internal fertilization, terrestrial species have direct development, marine species have indirect development (larval stages: trochophore, veliger)  importance of molluscs: food of animals and human, production of shells and pearls, are intermediate hosts (e.g. for liver fluke), are pests of agricultural plants  molluscs can be divided into 2 subphyla: Amphineura and Conchifera A) Subphylum: Amphineura − are marine and primitive − haven´t shell or have only shell made of organic substances − head isn´t well developed – without eyes and tentacles B) Subphylum: Conchifera − have shell made of calcium carbonate − we know 3 important classes of Conchifera: Gastropoda, Bivalvia, Cephalopoda 1) Class: Gastropoda − are asymmetrical − have spiral-coiled shell − head is clearly defined, separated from other parts of body, have eyes and tentacles − muscular foot can be modified for swimming or for burrowing − breathe with gills (some gastropods have pulmonary sacs) − are gonochorists or hermaphrodites (have complicated reproductive organs, gametes don´t mature in the same time – at first sperms, than eggs – this is called proterandrous hermaphroditism, two organisms change their sperms by copulation – sperms are stored in special shell until eggs are mature, then fertilization occurs) − lay eggs into the ground or stick them on some objects − live in ocean, fresh water and in terrestrial environment, too − have undergone torsion - the process does not occur in any other molluscs, it implies that the visceral mass and the mantle have become twisted through 180° in relation to the head and foot, as a result of torsion, all internal organs are twisted into a loop − members: Edible snail = Helix the most known, in pomatia (slimák some countries is záhradný) eaten White-lipped snail = common in Cepaea hortensis meadows and (slimák meňavý) gardens Slug - Agriolimax without shell agrestis (slizniak poľný) Bielzia coerulans without shell, (slizniak karpatský) endemic in Carpathian mountains, it´s blue  Great pond snail = in fresh water, Lymnaea stagnalis breathing with gills (vodniak vysoký) Great ramshorm = in fresh water Planorbarius corneus (kotúľka veľká) Viviparus viviparus eggs stay in the (močiarka živorodá) parent´s body until the end of embryonic development 2) Class: Bivalvia − are bilaterally symmetrical − are exclusively aquatic − lack a head − have a single shell composed of two valves, the valves are closed by two large transverse muscles − lacking a radula − breathe with gills − have sensory tentacles and photoreceptors (eyes) at the edge of the mantle − at the lower part of mantle there is mouth and anus, these openings can be prolonged to form tube-like siphons, siphon carries water to the mantle cavity (siphon is needed also for burrowing into substrate) − on foot they have a gland, which produces a secretion which is used for holding the substrate − most of them are filter feeders − are gonochorists, have larva veliger − used as food in places all over the world, pearl oysters are used for commercial production of pearls − members:  Painter´s mussel = Unio pictorum (korýtko rybničné) – our most common lotic species (living in flowing water)  Swan mussel = Anodonta cygnea (škľabka veľká) – our common lentic species (living in standing water) 3) Class: Cephalopoda − bilaterally symmetrical − exclusively marine − don´t have shell (or it is reduced – e.g as cuttelbone) − are carnivorous, feeding on crustaceans and fish (have radula) − mantle cavity is open through funnel-like opening, they force the water to flow through this opening out of the body very fast – it works like a pump, this type of movement is called jet-propulsion (tryskový pohon) system – it provides high- speed swimming with rapid direction changes, comparable to that of fish − foot is changed into tentacles (with suckers) − they have very simple cartilage – for attachment of muscles and for protection of “brain” − they breathe with gills − CS – closed − have the most complex nervous systems in the invertebrate world − they have ink sac – they release liquid similar to ink – for their protection, for escape − they have an ability to change skin colour due to surrounding − some of them are able to luminescence − have chamber eyes (similar to eyes of vertebrates) − are gonochorists, very often with sexual dimorphism, have hectocotylus – duct used to transfer spermatophores (sperm carriers) to female´s body, development is direct − are intelligent – are able to learn, have memory − members: Chambered nautilus = has shell composed of Nautilus pompilius many chambers – (lodienka) there is air – for floating in water Common cuttlefish = in warm seas Sepia officinalis (sepia obyčajná) Common octopus = common species Octopus vulgaris (osmonoh pobrežný)