Porifera-Reviewer

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- Also constitute prosopyles and apopyles of many syconoid and leuconoid sponges.

Cells present in the Mesohyl:

Archeocytes – (= progenitor cells) macrophage like; large amoeboid cells bearing a conspicuous nucleus and numerous large lysosomes.

- Totipotent and can differentiate into any other type of sponge cell.

- Phagocytic and play a role in digestion and internal transport.

Lophocytes – (= crest cells) archeocyte-like amoeboid cells that secrete collagen fibers from their trailing end as they move through the mesohyl.

- Produce and maintain the fine collagen fibers of the mesohyl.

Spongocytes – occur only in taxon Demospongiae

- Resemble archeocytes, but secrete collagen that polymerizes into thick skeletal fibers known as spongin.

Sclerocytes – (= hard shells) secrete the mineralized skeletal spicules of many sponges

Myocytes – (= muscle cells) muscle like cells containing actin and myosin that aggregate around the oscula of some demosponges

- Regulate the size of the oscular aperture and thus help to control water flow through the sponge.

Oocytes & Spermatocytes – reproductive cells that undergo gametogenesis in the mesohyl to form sperm and eggs.

Choanocytes – (= collar cells) flagellated collar cells of the choanoderm that generate the water flow through the sponge.

- Have an apical collar of long microvilli around a single flagellum.

- The collar is in the form of a cylinder or an inverted cone

- Basal part of the choanocyte flagellum (Microciona sp. & Grantia compressa) bears a bilateral vane, as in the choanoflagellates.

Syncytial

- the glass sponge’s body wall lacks the sheetlike pinacoderm pavement that covers the body and lines the aquiferous system of cellularian sponges.

Trabecular syncytium – living tissue in hexactinellids arranged in three-dimensional, cob-web-like strands or network

- cellular choanoderm is also absent.

Choanosyncytium – where the cellular choanoderm is placed.

Collar bodies – each with a collar and flagellum but lacking a nucleus, arise from the surface of the choanosyncytium.

* Each strand in the trabecular syncytium surrounds and encloses an axis of mesohyl.

- Mesohyl contains bundles of collagen fibers, spicules and cells – sclerocytes, archeocytes and presumably germ cells.

- The gross anatomy of hexactinellids is syconoid, but the aquiferous system, because it has both incurrent and excurrent canals, resembles the leuconoid design.

Water flow in Hexactinellids: Surface openings in the trabecular network – Incurrent canals – Collar-body chambers – Excurrent canals – Atrium – Osculum

WATER PUMPING

- In general, a sponge pumps a volume of water equal to its body volume once every 5 seconds.

- The flow of velocity is fastest through the osculum and slowest in the choanocyte chambers, because these two regions have, respectively, the smallest and the largest total cross sectional areas in the aquiferous system.

- Water current is produced by the activity of the choanocyte flagella

- Undulatory beat of each flagellum is in a single plane.

- The flagellar vane, which is restricted to the collar region of the flagellum , may help to pump water from the collar.

- The oscula of many sponges are situated on chimneys well above the main body and ostia – elevated position exposes the oscula to the environmental water currents faster than those that occur near the base of the sponge.

SKELETON

- the skeleton is chiefly a mesohylar endoskeleton, but an exoskeleton also may occur regionally or over the entire body.

- The mesohylar matrix matrix is supplemented with mineral spicules, sponging or both.

- In the extreme, a spicular skeleton can be a rigid, brittle, three-dimensional lattice or framework as in the glass sponge.

- The calcifying demosponges (“sclerosponges”) secrete a massive basal exoskeleton of CaCO3 on which the body rests. These sclerosponges also secrete siliceous spicules in the mesohyl.

- Some sponges lack spicules but secrete organic spongin.

- Spongin and spicules occur together in most species of sponges.

- Spicules are siliceous (SiO2) or calcareous (CaCO3) elements whose composition, size and shape are used at all levels in the classification of sponges.

- Megascleres – typically form the principal skeletal framework

- Microscleres – may support the pinacodermal lining of the canal system or in high density, toughen the body wall.

- A monaxon spicule has one axis and is shaped like a needle or rod, although it may be straight or curved, with pointed, knobbed or hooked ends.

- Triaxons have either three rays (triactines) or six (hexactines)

- Spicules are secreted extracellularly by sclerocytes in calcareous sponges, intracellularly in sclerocytes in demosponges and intrasynctically in glass sponges.

LOCOMOTION AND DYNAMIC TISSUES

- Both freshwater and marine species can move over a substratum at rates of 1 to 4 mm/day.

- Movement results from the collective amoeboid movements of pinacodermal and other cells.

- Other sponge movements: Whole-body contraction, constriction of oscula b myocytes.

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