The following points highlight the three main classes of Phylum Porifera. The three classes are: 1. Calcarea or Calcispongiae— (Calcareous Sponges) 2. Hexactinellida or Triaxonida or Hyalospongiae—(Glass Sponge) 3. Demospongiae.
Phylum Porifera: Class # 1. Calcarea or Calcispongiae— (Calcareous Sponges):
[Calcarea, L. Calcarious = limy, Calcispongiae, L. Calcis = genitive of calx = lime or chalk]
Features:
(i) Exclusively marine, shallow coastal water species, restricted to depth less than 100 metres and require hard substratum for attachment.
(ii) Small-sized sponges, about 10 cm in height.
(iii) Cylindrical or vase-like in shape.
(iv) Osculum narrow and placed terminally.
(v) Osculum provided with oscular fringe.
(vi) Comparatively large collared cells.
(vii) Skeleton represented by free calcareous spicules.
(viii) Spicules contain more CaCO3 (87%) than MgCO3 (7%) reported in Leucandra sp. and often differentiated into megascleres and microscleres. Organic matters in traces.
(ix) Megascleres are monaxon, triaxon or tetraxon.
(x) Canal system is asconoid, syconoid and leuconoid type. Asconoid type of canal system is found only in the class Calcarea.
The class Calcarea contains two orders.
Order 1. Homocoela:
Features:
(i) Asconoid sponges with small bodies.
(ii) Thin body wall and usually not folded internally.
(iii) Spongocoel is lined with choanocytes.
Typical examples of this order are Clathrina, Leucosolenia, Ascute, Ascyssa and Dendya.
Order 2. Heterocoela:
Features:
(i) Syconoid and leuconoid sponges, comparatively with large bodies.
(ii) Thick body wall and folded internally.
(iii) Only the radial canals are lined by choanocytes.
Typical examples are Sycon (= Scypha), Grantia, Leucandra.
Phylum Porifera: Class # 2. Hexactinellida or Triaxonida or Hyalospongiae—(Glass sponge):
[Hexactinellida, Gk. Hex = six, Gk. aktis = ray, L. ell – suffix added to form diminutives; Triaxonida, Gk. Treis = three, Gk. axon = an axle; Hyalospongiae, Gk. Hyaleos = glassy]
Features:
(i) Large sized sponge and on average 10 to 30 cm in height, live mainly in the deep waters of sea and can grow in firm and soft sediments. The deep sea forms live at the depths between 200 m and 1000 m.
(ii) Usually cup, vase or urn (vase with foot)-like shape.
(iii) Skeleton of six-rayed (triaxon) siliceous spicules (SiO2) or their modifications present either as separate entity or as networks.
(iv) Chemical analysis in Monoraphis reveals that the spicule contains SiO2 86%, water 9%, inorganic elements 3% and spiculin (a protein) 2%.
(v) Megascleres (skeletal spicules) and microscleres (flesh spicules) always distinguished.
(vi) Choanocytes restricted to finger-like simple or folded chambers.
(vii) Wall encloses a spongocoel (- atrium) which opens by a wide osculum.
(viii) Canal system may be either syconoid or leuconoid type.
(ix) There is no cellular dermal epithelium.
(x) Commonly called “glass sponge”.
Remark:
Some zoologists argue that the Hexactinellid sponges should be placed either in a separate phylum or in a subphylum under Porifera due to absence of cellular dermal epithelium and syncytial nature of outer body layer and inner flagellated layer (Pechenik, 2000).
It includes two orders:
Order 1. Hexasterophora
Features:
(i) The spicules are hexasters and never amphidiscs.
(ii) Radial canals or flagellated chambers are simple and lie radially in the sponge wall.
The typical example is Euplectella (Venus’s flower basket).
Order 2. Amphidiscophora:
Feature:
(i) The hexaster spicules are absent and the spicules are amphidiscs.
The typical examples are Hyalonema (Glass rope sponge), Pheronema (Bowl sponge).
Phylum Porifera: Class # 3. Demospongiae:
[Gk. demos = people + spongos = sponge]
Features:
(i) Mostly marine but a few are freshwater or brackish water forms. In sea they live from shallow water to great depths. 90% existing species fall under this class.
(ii) Brilliant colouration in most species, for the presence of pigment granules within amoebocytes.
(iii) Skeleton either absent or silicious (silicious spicules), fibrous (spicules replaced by organic collagenous fibres— spongin fibres, or both spongin fibres and siliceous spicules).
(iv) Silicious megasclere spicules never triaxon (6-rayed); microscleres are of different types
(v) Canal system of leuconoid type only. The leuconoid type canal system is derived from a larval stage, called the rhagon type which does not occur in any adult animals of calcareus sponges.
(vi) Flagellated chambers small and rounded.
(vii) Freshwater species of this class possess contractile vacuoles used for elimination of water from the cells.
(viii) Parenchymula larva in the life cycle of most demosponges.
It includes three subclasses and 7 orders.
Subclass 1. Tetractinellida
Features:
(i) Body rounded or flattened without branches.
(ii) Presence of tetraxon silicious spicules but the spongin fibres are absent.
(iii) In certain forms the spicules may be absent.
(iv) Shallow water forms.
It includes three orders:
Order 1. Myxospongida:
Features:
(i) Structure simple.
(ii) Skeleton or spicules are absent.
Examples are Oscarella, Halisarca.
Order 2. Carnosa or Homosclerophora or Microsclerophora:
Features:
(i) The megascleres and microscleres are not distinctly separable.
(ii) Spicules are all similar in size.
Examples are Plakina, Plakortis.
Order 3. Choristida:
Features:
(i) Spicules are long-shafted.
(ii) Megascleres and microscleres are distinctly differentiated.
The typical examples are Geodia, Ancorina, Craniella.
Subclass 2. Monaxonida:
Features:
(i) Body form varies from rounded mass to branching forms or stalked with funnel or fan-shaped.
(ii) Spicules are of monaxonial megascleres.
(iii) Spongin may or may not be present.
It includes four orders:
Order 1. Hadromerina or Astromonaxonellida:
Features:
(i) Megascleres are mostly tylostyles, i.e., broad end is knobbed.
(ii) Microscleres are usually wanting; when present, they are in the form of a star.
(iii) Spongin is absent.
The examples are Tethya, Cliona (Boring sponge), Poterion (Neptune’s goblet sponge).
Order 2. Halichondrina:
Features:
(i) Megascleres are always of more than one kind.
(ii) Microscleres are usually absent.
(iii) Spongin is very scanty.
The example is Halichondria (Crumb- of-bread sponge).
Order 3. Poecilosclerina:
Features:
(i) Megascleres are usually of two or more kinds and are localised.
(ii) Microscleres include the C-shaped, curved and bow-shaped types.
The examples of the order are Myxilla, Microciona.
Order 4. Haplosclerina:
Features:
(i) The megascleres are always diactinal, i.e., growth takes place at both directions and are not localised in distribution.
(ii) Microscleres may or may not be present.
(iii) Spongin is usually present.
Examples are Haliclona, (Finger sponge)), Chalina (Mermaid’s gloves sponge), Spongilla (Freshwater sponge), Ephydatia (Freshwater sponge).
Subclass 3. Keratosa:
Features:
(i) The skeleton is exclusively composed of spongin fibres.
(ii) The siliceous spicules are usually absent.
The examples are Spongia, Euspongia (Bath sponge), Hippospongia (Horse sponge), Phyllospongia (Leaf-shaped sponge).
Hartman and Goreau (1970) created a 4th class Sclerospongiae for some coralline sponges collected from caves and tunnels of coral reefs in Jamaica.
The features of this class are:
(i) A small number of species (about 15) of leuconoid sponges with silicious spicules and spongin fibres.
(ii) Secretion of a supporting mass of calcareous rock like matrix in addition to spicules of CaCO3, silica and spongin fibres.
(iii) Numerous spicules on their outside surface are slightly raised.
(iv) They are found in deep water.
Example. Astrosclera, Stromcitospongia, Hispidopetra.
Remark:
Recent morphological and molecular analyses suggest that the Sclerospongiae should be placed either in Calcarea or Demospongiae, but not in a separate class.