In this article we will discuss about the General Characters and Classification of Sponges.
General Characteristics of Sponges:
Some of the general characters of sponges are listed below:
1. Habitat:
All sponges are aquatic, mostly marine, rarely fresh water (e.g., Spongilla), solitary or colonial, sessile (attached to the substratum). Sponges like warmer water. They are not usually found in cold water.
2. Body Form:
Their body is porous, viz., provided with pores. The pores are of two types: inhalant pores are called Ostia (sing, ostium) and exhalent pores are known as oscula.
3. Symmetry:
Most of the sponges are asymmetrical. Some are radially symmetrical.
4. Germ Layers:
The sponges are the first multicellular diploblastic animals, i.e. derived only from two embryonic germ layers, viz., ectoderm and endoderm.
5. Level of Organization:
The sponges have cellular level of organization.
6. Body wall:
The body wall of a common sponge consists of three layers.
(a) Pinacoderm (= dermal layer):
It is outer cellular layer which consists of:
(i) Flattened pinacocytes and
(ii) Oporocytes (Fig. 4.7 & 4.8).
(b) Choanoderm (= gastral layer):
It is inner cellular layer which consists of highly specialized flagellated cells called choanocytes or collar cells (Fig. 4.7 & 4.8). Both pinacoderm and choanoderm do not have basement membrane.
(c) Mesohyl layer (= mesenchyme):
Basically, it is a non-cellular layer found in between pinacoderm and choanoderm. It has fine dispersed spongin fibres and numerous spicules (Fig. 4.7). It also contains amoebocytes (amoeba-like cells) of both pinacoderm and choanoderm.
Amoebocytes are modified into following cells (Fig. 4.8):
(i) Archaeoytes may be converted into other types of cells and are also called undifferentiated “totipotent” cells,
(ii) Trophocytes provide food to developing cells and are called nurse cells,
(iii) Thesocytes store food granules.
(iv) Gland cells secrete a slimy substance,
(v) Collencytes secrete spongin fibres of the mesohyl layer,
(vi) Scleroblasts secrete spicules. In calcareous sponges, they are called calcoblasts.
(vii) Myocytes form a circular ring around the osculum and help in closing and opening of the osculum.
(viii) Germ cells (Sex cells) form sperms and ova and develop during breeding season,
(ix) Chromocytes contain pigment granules and excretory substance,
(x) Phagocytes collect food from choanocytes through their pseudopodia and also engulf excreta and damaged tissues.
7. Canal System:
This system consists of pores and canals.
Three types of canal systems are found in sponges:
(i) Asconoid canal system. It is the simplest type which is found in Leucosolenia and a few other sponges,
(ii) Syconoid canal system. It is more complex than the ascon type. It is found in Sycon and some other sponges,
(iii) Leuconoid canal system.
It is most complex canal system which is found in Spongilla and some other sponges. In class Demospongiae the leuconoid condition is derived from a larval stage, called the rhagon. 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.
The central body cavity of a sponge is called spongocoel or Para gastric cavity. 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, excretory matter and reproductive bodies. Thus the canal system helps the sponge in nutrition, respiration, excretion and reproduction.
8. Skeleton (Fig. 4.7& 4.9):
Almost all sponges possess an internal skeleton. It may consist of calcareous or siliceous spicules or of fine spongin fibres or of both, located in the mesohyl layer.
9. Digestion:
It is intracellular and takes place inside food vacuoles as in protozoans.
10. Circulation:
Distribution of food from the ingesting cells to others is brought about by wandering amoebocytes of mesohyl layer.
11. Respiration:
Exchange of gases occurs by diffusion through the plasma membranes of the cells as in protozoans.
12. Excretion:
Removal of excretory matter also occurs by diffusion through the plasma membranes of the cells as in protozoans. Ammonia is chief excretory waste.
13. Reproduction:
Both asexual and sexual reproductions occur in sponges. Asexual reproduction occurs by budding and gem mules. In fresh water and a few marine sponges, gem mules or internal buds (Fig. 4.10) are formed. Sponges are hermaphrodite. Fertilization is internal.
14. Development:
Zygote undergoes holoblastic cleavage (complete division of the zygote). The development includes a free swimming larva, the amphiblastula (in Sycon) or parenchymula (in Leucosolenia) for dispersal of the species.
Unique features:
(i) Ostia and oscula present,
(ii) Presence of canal system and
(iii) Skeleton made up of spicules and spongin fibres.
Advancement over Protozoa:
(i) Multicellular body,
(ii) Division of labour among the cells,
(iii) Male and female gametes are sperms and ova.
(iv) Unicellular zygote undergoes cell division (mitosis) to form embryo.
Classification of Sponge:
Chiefly on the basis of skeleton, phylum Porifera (Sponge) is divided into three classes.
Class 1. Calcarea (L. Calcis- lime):
The skeleton is of calcareous spicules.
Examples: Leucosolenia, Sycon, (Scypha) Grantia.
Leucosolenia:
It is a simplest colonial sponge consisting of number of horizontal and vertical tubes. The development is with a larva, the parenchymula.
Sycon (= Scypha):
The development is with a larva, the amphiblastula.
Class 2. Hexactinellida (Gk. hexa- six):
The skeleton is of siliceous spicules which have six rays.
Examples: Euplectella, Hyalonema.
Euplectella (The Venus’ flower basket):
It is found in deep sea water. They are abundant near the Philippine Island and West Indies. Its skeleton is costly marriage gift in Japan as it is thought to be a symbol of union of wife and husband.
Hyalonema— The glass rope sponge:
Hyalonema is fixed in the mud by a root tuft of long, twisted spicules. The upper surface has the gastral cone which bears opening of the ex-current canals. The middle part of the root tuft commonly bears several polyps of symbiotic anemones of the genus Epizoanthus or Polythoa which show symbiotic relationship.
Class 3. Demospongiae. (Gr. demas- frame):
The skeleton is of spongin fibres or of spongin fibres with siliceous spicules or may be absent.
Examples: Euspongia, Spongilla, Cliona, Chalina.
Euspongia (Spongia) — The bath sponge:
The surface is raised into small projections, called the connules. Depressions between the connules bear small pores, the dermal ostea.The oscula are numerous: Its skeleton is used for bathing, washing automobiles, cleaning furniture and window panes. It is used for applying cosmetics, mopping, etc.
Spongilla (Fresh water sponge):
Spongilla is a common, widely distributed fresh water sponge shows various shades of green colour because of the symbiotic algae present inside.
Cliona (The Boring Sponge):
The sponge can penetrate the rocks and break them into pieces. The sponge can also bore through the oyster shell and, therefore, harmful for pearl oyster industry.
Chalina (The Dead man’s Fingers or “The Mermaid’s gloves”):
The sponge is popularly known as “the dead man’s fingers” or “the mermaid’s gloves”, because it is shaped like a hand with several fingers perforated with oscula.
