Phylum : Porifera

 PHYLUM: PORIFERA (Sponges) 

These are primitive, multicellular, asymmetrical (except Leucosolenia, Scypha) organisms having cellular level of organisation.

Most of them are marine and remain attached to rocks (sessile). A few live in fresh water e.g., Spongilla. This is the only phylum in animal kingdom without any nerve cells.

Important characters of phylum porifera 

1.    Body wall: The body wall of a common sponge consists of following layers: 

(a)    Pinacoderm (dermal layer) 

It is outer cellular layer which consists of (i) flattened pinacocytes and (ii) oval porocytes

(b)    Choanoderm (gastral layer) : 

It is inner cellular layer which consists of highly specialized flagellated cells called choanocytes (collar cells).

They are the characteristic cells of this phylum responsible for ingestion of food, secretion of mesohyal and differentiation of sex cells.

(c)    Mesohyl layer (mesenchyme) : 

Basically, it is a noncellular layer found between pinacoderm and choanoderm. It has fine dispersed spongin fibres and numerous spicules.

It also contains amoebocytes (amoeba-like cells) of both pinacoderm and choanoderm.

Amoebocytes are modified into the following types :

1. Archaeocytes. They may be converted into other types of cells and are also called undifferentiated "totipotent" cells.
2. Trophocytes. They provide food to developing cells and are called nurse cells.
3. Thesocytes. They store food granules.
4. Gland cells. They secrete a slimy substance.
5. Spongioblasts. They secrete spongin fibres of the mesohyllayer.
6. Scleroblasts. They secrete spicules. In calcareous sponges, they are called calcoblasts and in silicious sponges, they are called silicoblasts.
7. Collencytes. They secrete collagen fibers and form connective tissue.
8. Myocytes. They form a circular ring around the osculum and help in closing and opening of the osculum.
9. Germ cells (Sex cells). They form sperms and ova and develop during breeding season.
10. Chromocytes. They contain pigment granules and excretory substances.

(xi) Phagocytes. They collect food from choanocytes through their pseudopodia and also engulf excreta and damaged tissues.

2.    The central body cavity of a sponge is called spongocoel or paragastric cavity. 

3.    The continuous water current flowing through the canal system is very important for the life of a sponge. It brings in food and oxygen and carries away carbon dioxide, nitrogenous wastes and reproductive bodies. Thus the canal system helps the sponge in nutrition, respiration, excretion and reproduction. 

4.    Skeleton: Almost all sponges possess an internal skeleton. It may consist of calcareous or siliceous spicules or of fine spongin fibres or both, located in the mesohyl layer. 

5.    Digestion: It is intracellular and takes place inside food vacuoles as in protozoans. 

6.    Circulation: Distribution of food from the ingesting cells to others is brought about by wandering amoebocytes of mesohyl layer. 

7.    In case of Sycon, pinacoderm is divided into exopinacoderm and endopinacoderm. Exopinacoderm lines the incurrent canal and the spongocoel. Radial canals are lined by flagellated choanocytes. 

Concept Builder

What is luffa sponge? 

A luffa sponge isn't a sponge at all but a gourd. When dried, the fibrous material found in gourd forms a "skeleton" similar to that of some sponges. It can be used for the same purposes e.g., scrubbing. 
 

8.    Canal System: The body of a sponge is organized in such a manner as to form a complex system of pores and canals. This system is called canal system. It is meant for food gathering, respiration and removal of waste. 

• Three types of canal systems are found in sponges:

 (i)    Ascon type : It is the simplest type of canal system which is found in Leucosolenia and few other sponges. 

(ii)    Sycon type : It is more complex than the ascon type. It is found in Sycon and some other sponges. 

(iii)   Leucon type : It is most complex canal system which is found in Spongilla and some other sponges. 

In Demospongiae, the leuconoid condition is derived from a larval stage, called the rhagon. It is a conical organism with a flattened base. The canal system of rhagon larva does not occur in any adult sponge. Because of its derivation from rhagon stage in Demospongiae, the leucon type of canal system is also called the rhagon type. 

9.    Respiration: Exchange of gases occurs by diffusion through the plasma membranes of the cells as in protozoans. 

10.    Excretion: Removal of metabolic wastes also occurs by diffusion through the plasma membranes of the cells as in protozoans. Ammonia is chief excretory waste. 

11.    Reproduction: Both asexual and sexual reproduction occur in sponges. Asexual reproduction occurs by fragmentation, budding and gemmules. They are hermaphrodite and show internal fertilization. 

12.    Development: Zygote undergoes holoblastic cleavage (complete division). The development is indirect and includes a free swimming larva, the amphiblastula (in Sycon) or parenchymula (in Leucosolenia and other Porifers) for dispersal of the species.

CLASSIFICATION 

Based on the type of skeleton, phylum Porifera is divided into three classes.

Concept Builder

    Cultural Awarness 

1.    Using sponges: For centuries, people around the world have used natural sponges with spongin skeletons for cleaning, bathing by taking advantage of these sponges' soft flexible and highly porous bodies. 
        e.g., Euspongia. The ancient Greeks also used sponges as padding inside helmets. 

2.    Spongin fibres are elongated protein fibres which form a fibrous network. 

3.    Digestion in sponges is intracellular. 

4.    Regeneration in sponges was demonstrated by H.V.Wilson (1907). It is brought about by archaeocyte cells. 

5.    Sponge cells, seperated by straining pieces of sponge through a fine net, can reaggregate and grow into a sponge. So, a sponge is a republic of cells which identify one another, aggregate and grow together. 

 6.    Sponge reproduces asexually by fragmentation. During sexual reproduction, some cells become egg or sperm cells. After fertilisation, the zygote develops into a flagellated larva which swims, settles in a new place and grows into a sponge. 

7.    Proterospongia is a connecting link between protozoa and porifera. 

8.    In Hyalonema, root tuft consists of bundle of long anchoring spicules. These may pass through the columella (body axis) as gastral cone. It is commonly known as glass rope sponge. 

9.    Euspongia is commonly found is Mediterranean sea. It is commonly known as Bath sponge. 

10.    Amphiblastula is the hollow larva of Sycon etc. whereas parenchymula is the solid larva of most of sponges, e.g., Leucosolenia. 

Phylum Porifera

This animal group comprises Sponges and is thought to be the oldest on the planet. Sponges are the most basic multicellular creatures. They don't have any tissues or organs, despite being multicellular. Sponges need to be near water, thus they dwell in an aquatic environment. Water is essential for nutrient absorption, gas exchange, and excretion. The sponges' bodies have numerous holes or pores known as Ostia. Sponge bodies are built so that water can circulate through them, filtering out food and absorbing dissolved oxygen while also removing waste.This phylum of organisms lacks a specialized digestive, neurological, or circulatory system. Instead, they have a canal or water transport system that performs the activities of digesting, excretion, and gas exchange.

Their bodies are asymmetrical, and their design has been optimized to facilitate the most efficient flow of water via the core hollow that is present inside. Bacteria and other food particles found in the water are their main sources of nutrition. The spongocoel is a huge central hollow canal in their bodies. Water enters the spongocoel through the Ostia and exits through the osculum. Choanocytes or collar cells, with their flagellum jutting out, line up the spongocoel and canals.The water is moved throughout the sponge's body by the beating of this flagellum from all choanocytes. Common examples areSycon (Scypha), Spongilla (Freshwatersponge), and Euspongia(Bath sponge).