Porifera PDF

PHYLUM PORIFERA SPONGES, PORE-BEARERS Parazoa: Animals That Lack Specialized Tissues Amorphous, asymmetrical Sponges  Sponge is one of the simple metazoan A typical sponge lacks definite symmetry, growing as an irregular mass They lack germ layers and therefore have no tissues or or...

PHYLUM PORIFERA SPONGES, PORE-BEARERS Parazoa: Animals That Lack Specialized Tissues Amorphous, asymmetrical Sponges  Sponge is one of the simple metazoan A typical sponge lacks definite symmetry, growing as an irregular mass They lack germ layers and therefore have no tissues or organs. Sponge is a meshwork of cells surrounding channels that open to the outside through pores.  Being asymmetrical, there is no side referred to as anterior, posterior or oral surfaces 98% of the 15000 extant spp are marine while the 2% are found in freshwater Sponge contains several cell types, each with specialized functions; a layer of choanocytes, a layer of pinacocytes that form the pinacoderm, and amoeboid cells in the mesohyl between the two layers. This distinguishes sponges as truly multicellular, by contrast with colonial protists, which may form aggregates of cells, but all are functionally identical (except for the reproductive cells).  The zygote is totipotent i.e. has the capability of giving rise to all the kinds of cells in an animal’s body. In all animals except sponges, once a cell differentiates to serve a function, it and its descendants can never serve any other (irreversible) Sponges are unique in being able to dedifferentiate and then redifferentiate Dissociated cells can dedifferentiate to amoeboid form, re-aggregate and redeferentiate to reform a sponge  Sponge is a rigid perforated bag Body wall compose of outer epithelial-like pinacoderm made of pinacocytes The mesohyl contain amoebocytes and supporting elements Inner surface surrounding the spongocoel is lined by choanocytes SPONGE MORPHOLOGY  The pinacocytes also line the incurrent canals and spongocoel where choanocytes is lacking Pinacocytes contraction can vary the diameter of the ostia It also lead to minor or major shape change in sponge  Choanocytes flagella beating sucks water into the spogocoel through the ostia and expels it through the osculum Ostia are always numerous but there may be only one osculum. Porocytes that extend through the body wall form ostia. BODY WALL OF SPONGE  Choanocytes perform the following functions: Generate current that help circulate the sea water within and through the sponge Capture food particles Capture incoming sperm for fertilization  Water current passing through sponge perform the following functions: Sources of food particles System of gas exchange Removal of waste Gamete transfer  The gelatinous mesohyl layer though acellular and non-living contains : Amoeboid archaeocytes Supporting elements  Functions of the archaeocytes: Intracellular food digestion Store digested food materials Give rise to both male and female gametes Play active role in non-self recognition Elimination of wastes Secrete supporting elements Supporting elements (Or spicules/skeletal material) Calcareous or silicious spicules Spongin (A sulphur-containing proteinaceous material) Defenses is usually chemical, but spicules may discourage predation Regeration and Reproduction Sponge exhibit remarkable power of regeneration Asexual reproduction by gemmule formation and regression Gemmule is composed of archaeocytes cluster surrounded by capsule Gemmule is resistant to: Desiccation Freezing Oxygen lack Sponge Gemmule  Sexual reproduction is by gamete formation in the mesohyl Sperm exit the sponge through the osculum Embryonic development leads to formation of: Flagellated parenchymula larvae Amphiblastula larvae Poriferan Diversity (Body forms)  In Asconoid the spongocoel is lined by choanocytes The ostia is represented by intracellular canal Water enters directly through the ostia into the central spongocoel and leave through the osculum E.g. Leucosolenia  Asconoid imposes limitation on size The problem is solved by repeated folding of the flagellated layer to increase the surface area In syconoids the flagellated layer is evaginated at regular intervals into finger-like projections of radial canals Corresponding invaginations are called incurrent canals  Prosopyle forms connection between incurrent canal and radial canal or flagellated canals Apopyle is the opening of radial canal into spongocoel The flagellated canals of sycon sponge open into a central spongocoel E.g Scypha  Falgellated layer is increased in Leuconoid sponge by formation of many small chambers Water that enters dermal pore (ostia) passes a system of incurrent canals into the flagellated chambers Many leuconoids have no atrium (spongocoel) Water leaves the body through converging excurrent canals which opens at the osculum  Many leuconoids are irregular with many oscula on the body surface Increase in size in the leuconoid sponge comes with addition of large numbers of flagellated chambers Therefore leuconoid sponge attain the greatest size. E.g Spongilla  Classification Type, composition and morphology of the supporting elements is used in the classification sponges Four classes are recognizes: CALCAREA (Calcareous sponges) Spicules are usually separate and composed of calcium carbonate e.g Sycon, Leucosolenia  HEXACTINELLIDA (Glass Sponge) Spicules are siliceous and always include a six point spicule. Some spicules usually fuse to form long strand and highly organized skeleton. E.g. Euplectella  DEMOSPONGIAE Contains the greatest majority of sponges Skeleton composed of separate siliceous spicules Some possess skeleton of spongin Other members possess both spongin fibers and siliceous spicules  Body form is always leuconoid with irregular symmetry and many oscula E.g. Haliclona, Verongia, Cliona, Spongilla  SCLEROSPONGIA Small group of tropical sponges with siliceous spicules and sponging fibers They are encased within or resting on a solid external skeleton of calcium carbonate Body form is always leuconoid