In this article we will discuss about Sponges:- 1. Taxonomic Retrospect of Sponges 2. Definition and Origin of Sponges 3. General Features 4. Habitat 5. Structural Peculiarities 6. Development 7. Affinities 8. Regeneration 9. Phylogeny.
1. Taxonomic Retrospect of Sponges:
Up to 18th century sponges were not considered as animals. Ellis (1775) established the animal nature of the sponges. Linnaeus, Lamarck and Cuvier considered the sponges related to anthozoan polyps and included the sponges within zoophytes or polyps.Sponges were included under coelenterates throughout much of the nineteenth century, though de Blainville (1816) proposed to separate sponges from coelenterates and created a group Spongiaria having relationship with protozoa. R. E. Grant (1825) studied the morphology and physiology of sponges in greater details and named the group Porifera.
Huxley (1875) and Sollas (1884) wanted the sponges to be separated from higher Metazoa. These views were ignored till the middle of twentieth century, where they were treated as a separate side branch “Parazoa”. The sponges represent a parazoan grade of body construction where true embryonic germ layers are wanting.
Tuzet (1963) expressed the view that though sponges possess many primitive features (such as cellular grade of body organization, gas exchange, and response to external stimuli represent the unicellular protozoan-like animals), yet there is no doubt that they are in direct line of metazoan evolution.
Bowerbank and Norman (1882), Hyman (1940), Hartman and Goreau (1970, 75), Berquist (1978, 85), Barnes (1980), Meglitsch and Schram (1991), Anderson (1998) and Brusca and Brusca (2003) classified the porifera into 3 classes but a new class Sclerospongiae added to a few decades ago but was rejected few years later.
2. Definition and Origin of Sponges:
Porifera are asymmetrical or superficially radially symmetrical metazoa with cellular grade of organization without tissues, organs, and with a porous body and a canal system lined by choanocytes.
Origin:
Pre-Cambrian.
3. General Features of Sponges:
1. All are aquatic; mostly marine (98%) but a few are freshwater (Fam. Spongillidae).
2. They are sessile and sedentary animals.
3. Body either asymmetrical or some radially symmetrical in adult stage.
4. Multicellular organisms having cellular grade of organization without true tissues.
5. Body is perforated by numerous, minute inhalent pores—the ostia for ingress of water, hence the name of the group is called Porifera.
6. The wall of sponges consists of an outer epithelium, called pinacoderm, composing of flat, polygonal cells, the pinacocytes, and an inner single epithelial layer containing microvillous collard, flagellated choanocytes, called the choanoderm. Both the cell layers lack the basement membranes or basal lamina but occurs in other metazoan epithelia.
7. Presence of a middle gelatinous, non-living layer containing amoeboid cells and supportive skeletal elements, the layer is called the mesohyl or mesenchyme. The mesohyl corresponds to the connective tissue of other metazoans.
8. The interior of sponges has a single hollow cavity called the spongocoel or paragastric cavity lining the microvillous collard choanocytes in some and in majority of cases by folding of the wall of spongocoel, innumerable water canals form a complex structure (canal system) that drives water through the canals and conveys food and oxygen.
9. Body is strengthened by an internal skeleton of calcareous or silicious spicules or a collagenous fibres called spongin.
10. No definite organs and systems.
11. No mouth and gut. All are filter feeders. Digestion intracellular.
12. Muscles and nerve cells absent.
13. All sponges possess power of regeneration.
14. The cells of sponges show a high degree of independence.
15. Reproduction both asexual and sexual.
16. Asexual reproduction takes place by the fragmentation or through the production of gemmules or buds. Sexual reproduction by sperms and ova.
17. Mostly hermaphrodite.
18. Fertilization internal.
19. Cleavage complete and radial.
20. Absence of definite germ layers which are the most diagnostic feature of metazoans.
21. Two types of flagellated larval forms are seen in sponges-amphiblastula larva and Parenchymula (also called Parenchymella) larva.
4. Habitat of Sponges :
The Phylum Porifera includes nearly 8500 described species.
Most of them are marine excepting 150 freshwater sponges of the family Spongillidae. These sedentaric animals usually stay in low depths and use solid surface for fixation, but glass sponge reaches at greater depth and anchors on soft sediments. Cliona bores on the molluscan shell and is known as boring sponge. Sponges may be of varied colours and their shape depends upon the sites of their stay.
The largest sponge is Spheciospongia vesparum having a diameter of two metres. Certain sponges, e.g., Tethya can contract its entire body, while in most cases the contractility is restricted around the osculum. During unfavourable condition most sponges shrink and form restitution bodies, which grow in favourable condition.
In freshwater sponges specialised bodies, called gemmules, are formed for the same purpose. The gemmules remain viable for a period ranging from two months to three years. Many annelids and crustaceans live as symbionts with sponges. The body of sponge harbours many blue green and green algae. The well-known enemies of sponges are coral-reef fish, limpets and nudibranchs.
5. Structural Peculiarities of Sponges:
There are various types of sponges ranging from simple to complex. They have all in common certain structural features. The body is composed of loose aggregation of various types of cells (Fig. 11.10) which hardly form any tissue. There is no organ or organ system. They lack mouth and digestive cavity. Vital functions are performed by independent activities of the cells.
In simple asconoid sponges the wall is composed of an outer dermal epithelium or epidermis and an inner epithelium consisting of Choanocytes. A mesenchyme containing skeletal spicules and several types of free amoeboid cells are present between the epithelia.
The spicules support the body wall and hold the sponges erect. The mesenchymal cells originate from the outer epithelium, hence it may be considered as ectomesoderm. The body surface is perforated by pores acting for ingress of water.
Pinacocytes:
The surface of the body or epidermis is lined by pinacocytes. Each pinacocyte is a large flat polygonal cell. The central part of the cell is thickened due to the placement of nucleus. Pinacocytes may also line the spongocoel and incurrent canals of syconoid sponges and also the spaces in leuconoid sponges.
Such pinacocytes are sometimes referred to as endopinacocytes. Pinacocytes are highly contractile cells and can reduce the surface area of sponges. In many sponges a definite epidermis is absent as in Hexactinellida. It may form a syncytium in some cases.
Porocytes:
The pore cells or porocytes occur among the pinacocytes at frequent intervals. The porocytes are usually regarded as transformed pinacocytes but Prenant (1925) opines that the porocytes are the derivatives of amoebocytes. Porocytes are tubular cells extending from the epidermis to the spongocoel. They are pierced by a central canal which acts as an incurrent passage.
The cytoplasm of these cells contains many round inclusions. The porocytes are also highly contractile. The closure of the pore is effected by the advancement of a thin cytoplasmic sheet called the pore diaphragm from the margin to the centre at the outer end of canal.
Choanocytes:
Inner surface of the body is lined by specialised cells, the choanocytes. The collar cells or choanocytes are specialised cells with a rounded or oval base resting on the mesenchyme and a contractile transparent collar which encircles the base of a single long flagellum. These cells are much larger in size in Calcarea than in other sponges. Spicules are formed by the deposition of the scleroblasts.
Mesenchyme (= mesohyl):
The mesenchyme is commonly called the mesoglea or mesohyl. The mesoglea consists of a transparent gelatinous matrix of protein nature in which different types of cells like archaeocytes, amoebocytes, scleroblasts and germ cells are present.
The mesoglea is divided into two types:
(i) Collenchyme and
(ii) Parenchyme.
When the mesenchyme contains few cells—this is called collenchyma. The mesenchyme with many cells is designated as parenchyme.
Amoebocytes:
Amoebocytes are amoeboid in nature. They perform various functions and are responsible for producing other cell-types by the process of transformation excepting possibly the choanocytes. It is totipotent in nature. The amoebocytes are most important cellular entities in the life of sponges. The amoebocytes are of several varieties in different sponges.
The types are:
(i) Collencytes:
Amoebocytes with slender and branching pseudopods.
(ii) Chromocytes:
With lobose pseudopods and pigmented cytoplasmic inclusions.
(iii) Thesocytes:
With lobose pseudopods and many food reserves.
(iv) Scleroblasts:
Producing skeleton and subdivided into (a) Calicoblasts, (b) Silicoblasts and (c) Spongioblasts according to the nature of secreted skeleton.
(v) Archaeocytes:
The amoebocytes with blunt pseudopods, conspicuous nuclei with large nucleolus are referred to as the archeocytes. The archeocytes are actually the generalised amoebocytes which play the dominant role in regeneration, reproduction and differentiation of other cell- types.
Besides these cell-types long slender cells, called the desmocytes, are present specially in Demo- spongiae. Many fusiform contractile muscle cells or myocytes are present around the osculum.
6. Development of Sponges:
In most of the sponges the development takes place within the body of the parent but in a few of demospongiae the development of the fertilized eggs takes place in sea water. The cleavage is holoblastic and may be equal or unequal. In some calcareous sponges (e.g., Clathrina, Leucosolenia), Hexactinellida and most Demospongiae the embryos release as free-swimming coeloblastula (hollow blastula) stage (Fig. 11.14A).
The coeloblastula is oval shaped, single layer, with a central cavity. The outer single layer wall of the central cavity contains elongated monoflagellated cells except the posterior side where a few rounded granular non-flagellated cells, called archaeocytes, are present. Some of the flagellated cells loss their flagella pass into the central cavity and become amoeboid.
Gradually these amoeboid cells fill up the cavity, forming a stereoblastula (solid blastula) and differentiate into parenchymula (also called Parenchymella) larva (Fig. 11.14B). In most Demospongiae, the parenchymula larva develops directly from stereoblastula, having an external layer of flagellated cells and an inner mass of amoeboid cells, each cell contains single flagellum.
In some freshwater species, the parenchymula larvae possess spicules and choanocytes but in Hexactinellid species, the parenchymula larvae contain both spicules and chamber lined with choanocytes. After a brief swimming period they become attached by their anterior ends and develop into flattend plate-like structure with an irregular outline.
Most of the inner amoeboid cells migrate to the outer surface and form the dermal epithelium and mesenchyme. Gradually a cavity is developed and is lined by a layer of porocytes. With the increase of the size of cavity it is bounded by porocytes and flagellated cells. At last the cavity is lined by only flagellated cells, and porocytes change their position and come outside and form the ostia on the wall of sponges.
7. Affinities of Sponges:
Sponges represent the oldest form of metazoa. Fossil sponges were discovered from beds of Europe, Asia and North America, which are more than 600 million years old. The most widely known fossil genus is Archaeocyathus. No one questions the multicellular nature of sponges.
The close similarity with colonial protozoans like Proterospongia, unequivocally speaks about the protozoan affinity of porifera.
The protozoan affinity is attested by the following evidences:
(1) Absence of formed tissues;
(2) The presence of choanocytes;
(3) Mode of secretion of skeleton within single cell;
(4) Similar nature of nutrition;
(5) Existence of totipotency of cell-types; and
(6) Lack of cellular integration.
The sponges exhibit close protozoan affinities but the attempts to include sponges under the Phylum Protozoa failed because of the development of germ layers in developing sponges. Solas proposed to place the sponges under a subkingdom, the Parazoa, as an isolated branch of the Metazoa. Recent work have established that the sponges are metazoan of lower grade of organisation.
But their inclusion in the direct line of metazoan evolution has been thrown into doubt because they do not possess the following basic organisations of metazoa:
1. At morphological level:
The participating cells must remain arranged in layers and some amount of division of labour should be there. Certain amount of organisations like connective tissues, nervous tissues, are expected to be present even at the simplest form.
2. At physiological level:
In a true metazoa, each function should be carried by a group of cells. The outer layer of cells is primarily responsible for protection and inner layer of cells carries on the nutritive functions. There must be distinct differentiation of somatic and germinal parts of the body.
3. At embryological level:
A true metazoa will develop from a unicellular zygote and will pass through stages like blastula and gastrula. It is in the gastrula stage three primary germinal layers are differentiated and the blue print of future organisation is laid. Once differentiated, the germinal layers are irreversible in nature.
Unlike other metazoans reversal of germ layer takes place in sponges.
On the contrary, there are certain features in sponges which may be considered as unique to them.
These features are:
1. Within the sponge body each cell is an autonomous unit, i.e., each cell is independent and self-centred.
2. The osculum in sponge apparently resembles the mouth of coelenterates, but developmentally the osculum does not correspond with it.
3. The spongocoel corresponds to the coelenteron of the coelenterates but pores and network of choanocyte-lined canals are not seen in any metazoan group.
4. The mesohyl is poorly defined and contractility is restricted only to the region of the osculum.
5. The ability of amoeboid cells to become another cell-type speaks that in the group of sponges determination is not rigid like other metazoans
For this reason, the sponges were considered by L. H. Hyman as a blind lane from the high way of metazoan evolution and thus a new term “Parazoa” was coined to include them in a separate subdivision under the subkingdom Metazoa.
Such inclusion has recently been challenged by O. Tuzet, who after studying sponges for many years has again claimed that porifera has given rise to the true metazoans.
This idea takes following facts into consideration:
1. Choanocyte cells are seen in some echinoderms and therefore, are not the only characteristics of sponges. The flagellate cells in the inner lining of the cnidarians also bring a close resemblance.
2. The processes in gametogenesis, i.e., production of sperm and ovum, are same as in other metazoans.
3. The metazoans affinity obtained greater support from the study of spongin which has chemical and physical resemblance with the collagen.
4. The development of sponges like Oscarella shows similar processes as in metazoa. The cleavage of zygote results into a blastula stage (coeloblastula) which by invagination becomes gastrula.
5. The constituent cells exhibit less differentiation but are involved in several complicated organisations, i.e., formation of gemmules.
6. The unrestricted power of regeneration speaks about the primitiveness of sponges. The comparative study of regeneration has revealed that the power of regeneration decreases with the progress from lower to higher groups of animals. In most of the cases certain ’embryonic’ cells resembling the amoeboid cells of sponge play important role during regeneration.
Thus sponges are again proposed to be shifted in the high way of metazoan evolution and have been placed in between protozoa and cnidarians as phylum Porifera (Fig. 11.15).
The fact that the sponges evolved from the Protozoa and occupy a position between the Protozoa and Cnidarians is evidenced by the following arguments:
1. The choanocytes are diagnostic to the anatomy of sponges. Identical cells, as already stated, are present in many protozoans, echinoderms, annelids and molluscs.
2. The peculiar event in fertilisation that two synergids direct the sperm towards the egg is found to occur in chaetognatha.
3. The tetra-radial symmetry exhibited by the larvae of calcareous sponges is comparable with that of the larvae of polychaetes, sipunculids and gastropods.
4. Existence of wide regenerative power in sponges and in many lower metazoans.
5. The developmental dynamics of calcareous sponges show close parallelism with that of Volvox. This point strengthens the idea that the colonial flagellate protozoans hold the key to the evolution of the sponges.
8. Regeneration in Sponges:
Sponges can regenerate their lost parts very rapidly. H. V. Wilson (1907) for the first time demonstrated that a bit of sponge when squeezed through a silken mesh dissociates completely. Such dissociated cells, if kept under water, aggregate again and form the sponge body. Such property of sponge has later been seen by several workers in different marine and freshwater sponges.
The following incidences happen during re-aggregation of sponges:
1. At the dissociated stage, amoeboid cells (archaeocyte and amoebocyte) and flagellate cells (choanocytes) show movement (Fig. 11.16).
2. The archaeocytes and amoeboid cells (specially the archaeocytes) start to contact with each other and with other types of cells. This continues for 4-6 hours. At first, small aggregates of 4-8 cells are formed. Gradually small aggregates meet other cells or other similar aggregates and grow in size.
3. Within the aggregate, all the cells lose their identity and become homogeneous. This phenomenon is called de-differentiation (Fig. 11.17).
4. Gradually the peripheral cells of the newly formed aggregate form pinacocytes. This is followed by the appearance of flagellate cells, formation of spicules and establishment of canals within the aggregate.
5. It has been noted that excepting amoeboid cells, the other cell-types do not have the ability to be transformed into another type of cell, Thus from dedifferentiated state, pinacocytes, choanocytes and scleroblasts return to their original form. But amoeboid cells may be transformed into any other cell-type. For this reason amoeboid cells are regarded as totipotent cells.
9. Phylogeny of Sponges:
The sponges appeared during the Pre- Cambrian Period and a large number of fossils have been recorded from the Palaeozoic era up to recent. The evolutionary origin of sponges poses some interesting problems for their peculiar features.
The low level of cellular differentiation and presence of canal system, cellular totipotency and absence of tissues, basement membrane, body polarity, reproductive organs and functionally independent cells indicate a primitive stage of metazoan organization.
These characteristic features also suggest that sponges are phylogenetically remote from other metazoans. These features with monociliated flagella of sponges indicate that sponges have evolved directly from a protistan ancestor.
There are several views regarding the origin of sponges. Of which current view is that the sponges have evolved either from a simple, hollow, free-swimming colonial form or from a colonial choanoflagellate of protista. The choanoflagellates bear some features which are compared with the choanoflagellates (collar cells) of sponges.
But the collar cells are also found in certain other groups of invertebrates (e.g., in some corals, in the larval stages of some echinoderms). So the claim of the ancestry of sponges from a protistan choanoflagellate is almost refuted by the appearance of collar cells in other groups of metazoans
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