Phylum Porifera
The porifera or sponges represent the simplest group of multicellular animals. They have a few types of cells but are not organized into tissues and have no nervous system.
Mode of life:
Sponges are sessile benthonic animals that live fixed to the substrate. They are filter- feeders where organic particles are extracted from water entering through sponge pores. Waste products are expelled outside through the mouth (osculum).
BODY:
The body of sponge is a bag-shaped with a central cavity (paragaster) that open at the top via an osculum. The body consists of three layers; an outer ectoderm, an inner endoderm and a middle layer called mesoderm. The mesoderm secrets the skeleton (spicules) as well as reproductive organs. The outer surface of sponge is perforated by numerous pores (ostia) which lead to incurrent canals and then to chambers within sponge body. The chambers are lined with collar cells (choanocytes), these cells are flagellated where the flagella produce currents and thus allow water to enter sponge body.
SKELETON:
The skeleton of sponge consists of two types of material:
TYPES of spicules:
1) Monaxon: one- ray spine.
2) Triaxon: three- rays spine.
3) Tetraxon: four- rays spine.
4) Hexaxon: six- rays spine.
5) Desmas: the spicules have no regular shape.
Types of living sponge:
1-Ascon: the body is composed of a single chamber lined with choanocytes.
2-
Sycon: the body is composed of a number of grouped ascon-likechambers with a central opening.
3-
Leucon: the body is composed of a number of sycon-like chamberswhich open into the central cavity (paragaster).
Classification:
Sponges are divided into three main classes called
calcarea, hexactinellida and demospongia.1- Class: calcarea:
This class includes sponges having calcareous spicules (calcite or aragonite).
The spicules may be monaxon, triaxon or tetraxons. They may be loose or united together to form a rigid skeleton easily fossilized.
There is neither spongin nor silica. They extend from Devonian to recent
EX:
Raphidonema (Cretaceous).2- Class Hexactinellida:
This class includes sponges having siliceous spicules (composed of silica).
The spicules are Hexaxon and are large (megaloscleres) or small (microscleres). The spicules may be loose or united together forming a rigid skeleton. They extend from Cambrian to Recent and live today in the bathyal and abyssal zones.
The skeleton consists of spongin fibers alone, siliceous spicules alone (monaxon or tetraxon) or both spongin and siliceous spicules.
The spicules are of desmas type, which means irregular spicules formed by addition of silica to their rays (monaxon or tetraxon).
When desmas spicules unite together, they give rise to a rigid structure called lithistid, characteristic for lithistida which is an important group of fossil sponge extending from Cambrian to Recent.
N.B. Both Hexactinellida and Demospongia can be called Non-calcarea.
Stromatoporoids:
These are calcareous masses of layered and structured material found in carbonate rocks of Cambrian to Oligocene age. In thin section they show closely spaced laminae and rod-like pillars at right angle to each other. On the surface there are star-shaped grooves Astrorhizae similar to the exhalent canal system of sclerosponges. They were previously included in corals but the presence of spicules led paleontologists to consider them as sponges. They are important reef-builders in the Silurian and Devonian.
