B.Sc ZOOLOGY (HONS)

DEGREE I

PAPER 1

CLASSIFICATION OF PORIFERA

Dr.Anjali Gupta

Associate professor

Department of zoology

H.D.Jain College

Classification of phylum Porifera

Phylum Porifera are the lowest multicellular animals

belonging to the kingdom Animalia.

The word “Porifera” mainly refers to the pore bearers or pore

bearing species. Based on the embryological studies, sponges

are proved as animals and are classified into a separate

Phylum in the animals

This phylum includes about 5000 species. Poriferans are pore-

bearing first multicellular animals. The pores are known as

Ostia.

The Poriferans have a spongy appearance and are therefore

called sponges. They are attached to the substratum and do

not move. They have the ability to absorb and withhold fluids.

They were initially regarded as plants due to the green colour

and their symbiotic relationship with algae. Later, their life

cycle and feeding system were discovered and they were

included in the animal kingdom.

Characteristics of Phylum Porifera

Some of the important characteristics of phylum Porifera are

mentioned below.

1. The cells of Poriferans are loosely organized.

2. They are mostly found in marine water. Only a few are

found in freshwater.

3. They are either radially symmetrical or asymmetrical.

4. Their body is usually cylindrical.

5. The scleroblast secretes spicules while spongin fibres are

secreted by spongioblasts.

6. They have no organs in their body.

7. They depict cellular grade of organization.

8. The body is comprised of numerous pores known as

Ostia and osculum.

9. The central cavity is called spongocoel or atrium which

opens outside through the osculum.

10. They reproduce asexually by budding, and

fragmentation.

11. The nutrition is holozoic.

12. They have neurosensory cells but are devoid of any

specific nervous system.

13. They have the power to regenerate the lost parts.

14. The development is indirect and the cleavage is

holoblastic.

15. The exchange of respiratory gases and nitrogenous

wastes occur by the process of diffusion

Classification of Phylum Porifera

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.

1. Class: Calcarea

2. Class: Hexactinellida

3. Class: Demospongiae

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]

(i) Exclusively marine, shallow coastal water species,

restricted to depth less than 100 metres and require hard sub-

stratum 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:

(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:

(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.

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]

(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

(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:

(i) The hexaster spicules are absent and the spicules are

amphidiscs.

The typical examples are Hyalonema (Glass rope sponge),

Pheronema (Bowl sponge).

Class # 3. Demospongiae:

[Gk. demos = people + spongos = sponge]

(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

(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:

(i) Structure simple.

(ii) Skeleton or spicules are absent.

Examples are Oscarella, Halisarca.

Order 2. Carnosa or Homosclerophora or

Microsclerophora:

(i) The megascleres and microscleres are not distinctly

separable.

(ii) Spicules are all similar in size.

Examples are Plakina, Plakortis.

Order 3. Choristida:

(i) Spicules are long-shafted.

(ii) Megascleres and microscleres are distinctly differentiated.

The typical examples are Geodia, Ancorina, Craniella.

Subclass 2. Monaxonida:

(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:

(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:

(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:

(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:

(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:

(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.

*************************************************