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Chapter

BIO2231: Phylum Porifera characteristics

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Department
Biology
Course
BIO2231
Professor
Various
Semester
Spring

Description
PHYLUM PORIFERA: SPONGES Characteristics of Phylum Porifera: o Multicellular; body an aggregation of several types of cells differentiated for various functions, some of which are organized into incipient tissues with some integration o Body with pores (osita), canals, and chambers that form a unique system of water currents on which sponges depend for food and oxygen o Mostly marine; all aquatic o Radial symmetry or none o Outer surface of plat pinacocytes; most interior surfaces lined with flagellated collar cells (choanocytes) that create water currents; a gelatinous protein matrix called mesohyl contains amebocytes of various types and skeletal elements o Skeletal structure of fibrillar collagen (a protein) and calcareous or siliceous crystalline spicules, often combined with variously modified collagen (sponging) o No organs or true tissues; digestion intracellular; excretion and respiration by diffusion o Reactions to stimuli apparently local and independent in cellular sponges, but electrical signals in syncytial glass sponges; nervous system probably absent o All adults sessile and attached to substratum o Asexual reproduction by buds or gemmules and sexual reproduction by eggs and sperm; free-swimming flagellated larvae in most The Origins of Multi-cellularity: While sponges are multicellular, their organization is quite distant from other metazoans. A sponge body is an assembly of cells embedded in an extracellular matrix and supported by a skeleton of minute needle- like spicules and protein. Origin of Metazoa (Animals): Choanoflagelletes are solitary/colonial aquatic eukaryotes, with each cell carrying a flagellum surrounded by a collar of microvilli. Beating of the flagellum draws water into the collar, where microvilli collect tiny particles. Many choanoflagelletes are sessile and attached to hard surfaces. Choanoflagellete cells strongly resemble sponge feeding cells called choanocytes. Adult sponges have simple bodies; they are aggregations of several different cell types, including choanocytes, held together by an extracellular matrix. Most sponge bodies are not symmetrical, but some appear radial. A sponge body has neither a mouth nor a digestive system. Phylum Porifera: Sponges: Sessile sponges draw food and water into its body instead. A sponge uses a flagellated ‘collar cell’, the choanocytes to move water. The beating of many tiny flagella, one per choanocyte, draws water past each cell, bringing in food and oxygen, as well as carrying away wastes. The sponge body is designed as an efficient aquatic filter for removing suspended particles from the surrounding water. Although their embryos are free swimming, adult sponges are always attached. The skeletal framework of a sponge can be fibrous and/or rigid. When present, the rigid skeleton consists of calcerous or siliceous support structures called spicules. The fibrous part of the skeleton comes from collagen protein fibrils in the intercellular matrix of all sponges. Collagen comes in several types of differing in chemical composition and form. One form of collagen is spongin. The presence of photosynthetic organisms inside the sponge led some to propose that spicules were able to transmit ight into the body. 3 classes of porifera: - Calcera - Hexactinellida - Demospongiae Calcera have spicules of crystalline CaCO3 with 1, 3 and 4 rays. Hexactinellida are glass sponges with 6-rayed siliceous spicules, where the 6 rays are arranged in 3 planes at right angles to each other. Demospongiae have a skeleton of siliceous spicules that develop around an axial filament, or sponging fibers, or both. A fourth clase, Homoscleromorpha, contains sponges that lack a skeleton/have siliceous spicules without an axial filament. Form and Function: Sponges feed primarily by collecting suspended particles from water pumped through internal canal systems. Water enters canals through a multitude of tiny incurrent pores in the outer layer of cells, a pinacoderm. Incurrent pores = dermal pores. Inside the body, water is directed past the choanocytes, where food particles are collected on the choanocytes collar. The collar comprises many fingerlike projections, called microvilli. The use of the collar as a filter is one form of suspension feeding. The smallest particles, accounting for about 80% of the particulate organic carbon, are taken into choanocytes by phagocytosis. Choanocytes may acquire protein molecules by pinocytosis. Two other cell types, pinacytes and archaetocytes, play a role in sponge feeding. There are 3 main designs for a sponge body, differing in placement of the choanocytes. In the simplest as asconoid system, the choanocytes lie in a large chamber called the spongocoel. In the syncoid system, the choanocytes lie in canals. In the leuconoid system, the choanocytes occupy distinct chambers. Types of Canal Systems: Asconoids: Asconoid sponges draw water inside through dermal pores by the beating of large numbers of flagella on the choanocytes. These choanocytes line the internal cavity called the spongocoel. As the choanocytes filter the water and extract food particles from it, used water is expelled through a single layer osculum. This design has distinct limitations because choanocytes line the spongocoel and can collect food only from water directly adjacent to the spongocoel wall. Asconoids are small and tube-shaped, because if the spongocoel were large, most of the water and food in its central cavity would lie in ‘dead cavity’ inaccessible to choanocytes. Asocnoids occur only in class Calcera. Synconoids: Synconoid sponges have a tubular body and single osculum, but the body wall, which is really the spongocoel lining, is thicker and more complex than that of asconoids. The lining has been folded outward to make choanocyte-lined canals. Folding the body wall into canals increases the surface area of the wall and thus, increases the surface area covered by choanocytes. Most of the water in a canal is accessible to choanocytes. Water enters the syconoid body through dermal ostia that lead into incurrent canals. It then filters through tiny openings, or prosopyles, into the radial canals. Here food is ingestedby the choanocytes. The beating of the choanocytes’ flagella forces the used water through internal pores, or apopyles, into the spongocoel. Food capture does not
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