Phylum porifera is classified mainly basing on skeleton. It is divided into 3 classes. sponges spicules plays a very important role in classification of porifera. Class: Calcarea Class: Hexactinellida Class: Demospongiae CLASS I : CALCAREA: Skeleton is made by calcareaous spicules. They show single spongocoel cavity which opens to the outside through a single opening called osculum. Radially symmetrical animals. Choanocyte cells are large. This is divided into 2 orders. Order 1 : Homocoela: 1. Spongocoel is lined by choanocytes internally. 2. Simple sponges or included in this order. Ex: Clathrina, Leucosolenia, Olynthus. Order 2 : Heterocoela 1.Spongocoel is lined by epithelial cells. 2.Choanocytes restricted to radial canals only. 3. Leuconoid or Syconoid canal system is seen. Ex: Sycon, Grantia. CLASS II : HEXACTINELLIDA (OR) HYALOSPONGIAE: Skeleton contains six rayed, or triaxon, siliceous spicules. Radially symmetrical. These are also known as glass sponges. This class includes 2 orders Order 1. Hexasterophora : 1. The spicules are of hexasters type. 2. They are attached to hard objects. Ex: Euplectella Order 2. Amphidiscophora : 1. The spicules are amphidiscs. 2. These sponges are attached to the substratum by root tufts. Ex: Hyalonema, Glass rope sponge CLASS III : DEMOSPONGIAE: Skeleton either absent or present. The skeleton contains spongin fibres or silicious spicules or both. This class is divided into 3 subclasses. Subclass 1. Tetractinellida: The skeleton consists of tetraxon spicules or without spicules. They are mostly solid rounded structures. Leuconoid canal system is present. This subclass is divided into three orders. Order 1. Myxospongida: 1.Structure is simple 2.Skeleton is absent. Eg. Oscarella Order 2. Carnosa (or) Homosclerophora: 1. Equal sized spicules are present. 2. Megascleres and microscleres are not sharply differentiated. Ex: Plakina. Order 3. Choristida: 1. Microscleres and megascleres are distinct. Eg. Tetilla, Geodia. Subclass 2. Monaxonida: Skeleton consists of monaxon spicules. Spongin fibres may or may not be present. They are cosmopolitan in their distribution. Shallow or deep water forms. This subclass includes 3 orders. Order 1. Hadromerida: Spongin fibres absent.Monaxon spicules are represented by tylostyles. Eg. Cliona (Boring sponge), Tethya. Order 2. Halichondrida: Monaxon spicules are seen. Eg. Halichondria. Order 3. Poecilosclerida: Monaxon spicules are present. Microscleres are sigmas, toxas etc. Eg. Microciona.