In this article we will discuss about Scypha:- 1. History of Scypha 2. Habit, Habitat and Distribution of Scypha 3. External Features 4. Canal System 5. Histology 6. Skeleton 7. Water Current 8. Nutrition 9. Respiration 10. Excretion 11. Nervous System and Behaviour 12. Reproduction.
History of Scypha:
Scypha (Gr., skyphos = cup) was formerly called Sycon. According to de Laubenfels (1936) the name Sycon must be replaced by Scypha. Scypha is somewhat more complex type in comparison to Leucosolenia because Leucosolenia is primitive asconoid type without any folding in its body wall; the body wall of Scypha is somewhat folded and, therefore, its spongocoel is comparatively reduced.
In fact, due to various degrees of folding in the body wall, the organisation of such sponges varies much. Scypha, however, represents first stage of folding in its body wall having finger-like horizontal folds, referred to as typical syconoid type. Grantia, an European genus, is another syconoid sponge.
Habit, Habitat and Distribution of Scypha:
Scypha, also known as crown sponge, is a small, marine sponge found attached by a sticky secretion to some submerged solid object like rocks, shells of molluscs and corals. It is found in shallow water up to a depth of 50 fathoms (1 fathom = 6 feet) where waves provide the animal with plenty of food and well oxygenated water.
It is a branching colonial sponge, though solitary individuals are also found. Scypha is widely distributed and found in abundance near North Atlantic shores. The different species of Scypha are S. ciliatum, S. elegans, S. coronata, S. lingua, S. gelatinosum and S. raphanus.
External Features of Scypha:
Scypha is vase-shaped and is 2.5 to 7.5 cm in length. It has several cylinders, all the cylinders are connected at the base by which it is attached by a sticky secretion to some submerged solid object in the sea. It is grey or light brown in colour. The distal or free end of each cylinder has a single large opening, the osculum or exhalent or ex-current pore.
The osculum is encircled by an upstanding collar of long monaxon spicules termed the oscular fringe looking like a crown, hence, the name crown sponge is given to it. It prevents the entry of other animals into the sponge, Below the osculum is a short, narrow collar region.
The body of the sponge is covered externally by a thin dermal epithelium or ectoderm. The surface of a cylinder has polygonal elevations, and between the elevations are depressed lines, in the depressions are groups of ostia which are inhalant or incurrent pores. These are intercellular apertures and not intracellular as in Leucosolenia.
Inside each cylinder is a spongocoel (Gr., spongos = sponge; koilos = hollow) or Para gastric cavity which is not digestive cavity. The wall of the cylinder is thick due to increase in the amount of mesogloea, the wall has folded in such a way as to form two types of canals, the incurrent canals and radial canals, they lie alternately and radially around the spongocoel,but ostia and canals are absent from the collar and basal regions.
Canal System of Scypha:
Scypha, like all other sponges, possesses the characteristic peculiarity-the canal system. The sponge body is traversed by numerous canals of several types’ which together form the canal system. It plays a very important role in the life of Scypha like those of other sponges. The particular type of canal system found in Scypha is known as syconoid type which is more advanced than the asconoid type.
It consists of the following:
(i) Spongocoel:
If the cylindrical body of Scypha is cut open longitudinally, it shows that osculum leads into a narrow tubular cavity called the Para gastric cavity or spongocoel. The spongocoel is lined with thin, gastral ectodermal cells called, pinacocytes.
(ii) Radial or Flagellated Canals:
The thick body wall lining the spongocoel contains many finger-like, straight outpushings at regular intervals called radial or flagellated canals. The radial canals are lined with flagellated collar cells or choanocytes. Each radial canal appears octagonal in cross section.
The radial canals are closed at their outer ends towards the surface of cylinder but at their inner ends each communicates with a small wide ex-current canal by an aperture called apopyle (Gr., apo = away from; pyle = gate) or internal ostia which joins the spongocoel.
(iii) Ex-Current Canal:
The ex-current canal is a short and wide passage between the radial canal and spongocoel. It leads radial canal into the spongocoel. The ex-current canals are lined with flat ectodermal cells like the spongocoel. The broad connection between the ex-current canal and the spongocoel is called the gastric ostium.
Between the radial canal and ex-current canal is a thin diaphragm perforated by large hole called apopyle. The apopyle is surrounded by contractile myocytes (Gr., myos = muscle; kytos = cell) due to which it can contract or dilate.
(iv) Incurrent Canals:
In between the two successive radial canals, a tubular space called incurrent canal is present. Thus, radial canals and incurrent canals are arranged alternately.
The incurrent canals are narrow passages somewhat square in section. The incurrent canals open to the exterior between the blind outer ends of the radial canals by apertures termed dermal ostia (L., ostium = door) or dermal pores. Incurrent canals are lined by flat ectodermal cells called pinacocytes like the spongocoel.
Externally an incurrent canal is covered by a pore membrane pierced by 3 or 4 ostia which are intercellular (in Leucosolenia the ostia are intracellular). The incurrent canals end blindly, at their inner ends, not reaching the spongocoel. Between the incurrent canal and spongocoel, the mesogloea is specially thickened to form the gastral cortex.
The incurrent and radial canals run side by side. The wall between the incurrent and radial canals is pierced by numerous minute pores called prosopyles (Gr., pros = near, pyle = gate). The prosopyles are perforations in porocytes. The porocytes are tubular contractile cells formed from modified pinacocytes.
The circulation of water in Scypha takes place in the following way. The water enters into the incurrent canals through the ostia, passes into the radial canals through prosopyles and from radial canals into the spongocoel by apopyles and leaves the spongocoel by terminal osculum.
In other way, the course of water into the canal system can be represented as water from outside → dermal ostia → incurrent canals prosopyles → radial canals → apopyles → spongocoel → osculum → outside.
Histology (Microscopic Structure) of Scypha:
All sponges are diploblastic, i.e., their body wall is composed of two layers with an intermediate mesenchyme (Gr., mesos = middle; enchyme = infusion) or mesohyl. The pinacoderm consisting of exopinacoderm covering the body surface except ostia and osculum and endopinacoderm lining the incurrent canals and spongocoel.
It is made of thin, large and polygonal, scale-like pinacocytes. Pinacocytes are ectodermal in origin and lie with their edges touching so that they form a loose layer and also line the incurrent canals.
The choanoderm forming gastral epithelium, lining the radial canals, consists of single layer of large choanocytes or collar cells which are endodermal in origin and were discovered by James Clark (1867).
The choanocytes are rounded or oval cells whose base rests upon the mesenchyme, while the free end bears a transparent contractile collar encircling the base of the single long flagellum. A choanocyte also contains a nucleus, a contractile vacuole and food vacuoles.
The electron microscopic structure of a choanocyte reveals the presence of all cellular organelles like mitochondria, Golgi apparatus, endoplasmic reticulum, ribosomes, etc., in it. Its collar is formed by 20-30 cytoplasmic processes called microvilli which are contractile in nature.
The microvilli are often connected together side by side. The flagellum consists of usual pattern of fibrils in 9 + 2 and originates from basal granule or kinetosome. The choanocyte cell has root-like processes embedded in the mesenchyme.
Between the dermal epidermis and endoderm is an intermediate layer, the mesenchyme. The mesenchyme consists of a gelatinous transparent matrix, commonly called mesogloea or mesohyl, presumably of a protein nature, in which free amoeboid cells or amoebocytes wander about.
When there are much mesogloea and relatively few cells, the mesenchyme is termed collenchyma; when the cells are numerous, the name parenchyma is applied. The mesenchyme is like a gel and holds the spicules in place.
Sponge Cells or Cellular Elements of Scypha’s Body:
There are several types of cells found in sponges which are not organised into well defined tissues, the cells form loose collections and they act more or less independently of each other.
The sponge cells are as follows:
1. Pinacocytes are thin polygonal scale-like cells, they are large and flat with a central nucleus, they lie with their edges touching, they are highly contractile. Contractions or expansions of the edges of pinacocytes can slightly decrease or increase the size of the entire sponge.
Pinacocytes form the external dermal layer, they line the incurrent canals, and in some they also line the spongocoel. The external pinacocytes are spoken of as “ectoderm” and those lining the spongocoel as “endoderm”.
2. Porocytes or pore cells are modified pinacocytes, they are large-size contractile cells; through the porocyte runs a large perforation called prosopyle which connects an incurrent canal to a radial canal or to a flagellated chamber.
3. Choanocytes or flagellate endoderm cells are large, oval, nucleated cells, each with a contractile vacuole and some food vacuoles; at one end is a long flagellum arising from a basal granule, the basal granule is joined to centriole and the two together are called a centroblepharoplast which controls the movements of the flagellum. Below this is a parabasal body joined by a fibril to the centroblepharoplast and the nucleus.
The flagellum has two central fibrils and nine double peripheral fibrils, it is surrounded at its base by a transparent contractile collar made of a ring of erect and closely set microvilli which project from the cell. Choanocytes form a layer of adjacent cells in radial canals or flagellated chambers of sponges, the movement of their flagella causes a current of water. Choanocytes are endodermal in origin.
4. Amoebocytes are amoeboid cells with pseudopodia, they wander about freely in the mesogloea, they are modified to form the following types of cells:
(i) Collencytes have several slender, long, branching pseudopodia, the branches are like connective tissue cells in the mesogloea.
(ii) Archeocytes are large amoebocytes, they have a few blunt pseudopodia, the nucleus is large, they are generalized cells and they transport food and waste substances. They can give rise to other types of amoebocytes, and they form sperms, eggs and asexual reproductive bodies called gemmules, such cells which can change into almost any kind of cell within an animal are said to be totipotent.
(iii) Chromocytes are pigmented amoebocytes having lobose pseudopodia. They probably impart colouration to the body of sponge.
(iv) Thesocytes are amoebocytes having lobose pseudopodia like chromocytes but filled with food reserves. Thus, these cells work as storage cells.
(v) Trophocytes are found loaded partly or completely with digested food and serve to transfer it; from one place to other.
(vi) Phagocytes collect food from choanocytes through their pseudopodia and also engulf excreta and damaged tissues.
(vii) Myocytes are fusiform contractile cells, they form a sphincter around apertures, such as oscula and apopyles where they act like muscles to open or close these apertures, they show some similarity to involuntary muscles in shape and contractility.
(viii) Scleroblasts are amoebocytes in the act of secreting the skeleton of a sponge. They are called calcoblasts if they secrete calcareous spicules, or silicoblasts if they secrete siliceous spicules, or spongioblasts if they secrete spongin fibres.
(ix) Gland cells are amoeboid with a long strand at one end, they are found attached by their strands to the surface of a sponge, they secrete slime.
(x) Germ cells are male and female gametes (sperm and ova)differentiate from archeocytes cells. In some cases, the germ cells are said to be differentiated from choanocytes.
Skeleton of Scypha:
In Scypha the skeleton consists of calcareous spicules. The spicules or sclerites are definite bodies, having a crystalline appearance and consisting in general of simple spines or of spines radiating from a point. They have an axis of organic material around which is deposited the inorganic substance either calcium carbonate or hydrated silica. Spicules present a great variety of shape.
In Scypha the spicules have a definite arrangement with regard to its canal system and are of the following types:
(i) Large one rayed needle-like monaxon spicules are arranged in a circlet around the osculum.
(ii) Simple spear-like monaxon spicules project from dermal cortex opposite the outer ends of the radial canals.
(iii) Three rayed or triaxon spicules are present along the radial canals with their one end pointing towards the distal ends of the canals.
(iv) Four rayed or tetraxon spicules are present, along with triaxon spicules in the thick gastral cortex surrounding the spongocoel. In the body of Scypha, the monaxon spicules project from the body surface and may be needle-like or spear-like, while the triradiate spicules lie embedded inside forming a network.
On the outer surface the monaxon spicules project in masses from the polygonal elevations where they partly conceal and protect the ostia, each group of these spicules is collectively called oxeote spicules.
Water Current of Scypha:
Water current plays a very important role in the physiology of the sponges. The current of water brings in food and oxygen and it takes away waste products. Under normal conditions all the apertures of a sponge are widely open and a current of water flows through the animal and out at the oscula.
The water current is caused by the beating of the flagella of the choanocytes, but as the flagella do not beat in coordination, the way in which a current of water is produced is not clearly understood. The most plausible explanation is that of Van Tright based on observations of thin expansion of freshwater sponges.
The flagellar movement consists of a spiral undulation passing from base to tip and creating a water current in the same direction.
As the choanocytes in each flagellated chamber are grouped near the prosopyle with their collars more or less pointed towards the apopyle, the water currents tend to flow from the flagellar tips towards the apopyle. The mechanism will obviously be more effective when the apopyles are larger than prosopyles, as is usually the case in sponges, since the water will tend to seek the larger outlet.
The water current enters the body of Scypha through its ostia into the incurrent canals, then through the prosopyles it goes to the radial canals lined by choanocytes. From the radial canals the water enters the ex-current canals through the apopyles from where it enters the spongocoel, then the water goes out through the large osculum.
Nutrition of Scypha:
Scypha feeds on particles of organic matter and small living organisms, such as bacteria, diatoms and Protozoa, they are drawn in with the water current. Food is ingested partly by porocytes but mainly by choanocytes either through the side of the cell or through its collar. In the cell a food vacuole is formed in which digestion occurs. Digestion is entirely intracellular, as in Protozoa.
The contents of the food vacuoles are first acidic, then they become alkaline. Several enzymes have been identified in sponge extracts. Partly digested food is taken up by amoebocytes in which digestion is completed, the amoebocytes transport and supply the digested food to all parts of the body.
Digested food is stored as reserves, chiefly glycogen, fat and glycoproteins, and lipoproteins in amoebocytes which are termed as the socytes.
Undigested remains of food are cast out through the collars of choanocytes from where they pass out with the current of water. But in non-calcareous sponges the food is transferred from choanocytes to amoebocytes or amoebocytes engulf the food directly, digestion v occurs only in amoebocytes which also egest the undigested particles.
Respiration in Scypha:
Special respiratory organs are wanting in sponges. Gaseous exchange occurs by simple diffusion, between the cells of sponge and the current of water. Oxygen dissolved in water is taken in by diffusion through the general body surface by the pinacocytes and internally by the choanocytes. Amoebocytes distribute the oxygen throughout the mesenchyme and take away the carbon dioxide.
The process of respiration is entirely intracellular as in Protozoa. Sponges prefer places where water contains plenty of oxygen. If kept in foul water or water deficient in oxygen contents or if their dermal pores become clogged with silt,the sponges die or undergo reduction.
The rate of consumption of Scypha was found to range from 0.16 ml. of oxygen per gram of fresh weight per hour in the smaller specimens to 0.04 in the larger ones. The upper half just above the osculum consumes 10 to 50 per cent more oxygen per gram per hour than the basal half.
Excretion in Scypha:
Egested wastes and excretory matter (largely ammonia) leave the body with the current of water. Some observers claim that amoebocytes containing excretory granules and inclusions are discharged by sponges.
Nervous System and Behaviour of Scypha:
Sponges are devoid of sensory or nerve cells. The sponge body displays only very slight powers of conductivity. Strong stimuli such as cuts or sharp blows are transmitted not at all or only 3 or 4 mm at the most. Conductivity is the best developed at the osculum where transmission occurs more readily away from than toward opening. The oscular rim appears to be the most sensitive part of the sponge.
However, Tuzet (1953) and Pavans de Ceccatty (1955) believe that the collencytes are nervous in function; some of them behave like neurons and constitute a diffused nerve net connecting the pinacoderm, choanoderm and myocytes. Thus, these cells acting as neurons are believed to receive and conduct the various types of stimuli.
Reproduction of Scypha:
Scypha reproduces both asexually and sexually.
(i) Asexual Reproduction:
In Scypha asexual reproduction takes place by budding and regeneration.
(i) Budding:
A small bud appears at the base of an adult Scypha and grows into full size. It may adhere to the parent and, thus, help to form a colony, or it may break free and forms a new individual and leads an independent life.
(ii) Regeneration:
Regenerative power of sponges is very high. Any piece of the body of Scypha is capable of growing into a complete sponge, if kept in a suitable environment.
(ii) Sexual Reproduction:
Scypha is monoecious (hermaphrodite). Both sperms and ova are produced from the archaeocytes which are present in the mesogloea.
Oogenesis:
The egg mother cell or ovocyte is first derived from an enlarged amoebocyte with a large nucleus and conspicuous nucleolus.
It grows and acquires food stores by engulfing or fusing with other similar amoebocytes or may receive supplies from special trophocytes. Upon attaining full size it undergoes the usual maturation divisions to form the ovum which lies in the wall of the radial canal, ready to be fertilised by a sperm from another sponge.
Spermatogenesis:
The sperm mother cell or spermatogonium is described as an enlarged amoebocyte that soon becomes enveloped by one or more flattened cover cells derived by the division of the mother cell or consisting of other amoebocytes. The whole is called spermatocyst.
The spermatogonium divides two or three times into the spermatocytes which give rise to sperm. The sperm comprises a rounded nucleated head and vibratile long tail. Other authors state that spermatogonia are transformed choanocytes, and Gatenby has described the transformation of an entire flagellated chamber into spermatozoa.
Fertilisation:
Due to protogynous condition cross fertilisation occurs which is internal and the eggs are fertilised in situ.
The sperm does not enter directly in the ovum but reaches a radial canal and is dispersed by the water currents. The sperm enters a choanocyte adjacent to ripe ovocyte. The choanocyte loses its collar and flagellum, becomes amoeboid, and plasters itself to the surface of the ovocyte, which forms a conical depression to receive it.
The sperm in the meantime has lost its tail, and its swollen head becomes surrounded by a capsule. The capsule carrying the sperm head penetrates into the ovocyte. According to Gatenby and others the sperm enters a choanocyte, which acts as a nurse cell and then fuses with the egg but according to Duboscq and Tuzet (1937) the sperm carrying choanocyte departs after the transfer of the sperm into the ovocyte.
Early Embryonic Development:
The fertilised egg undergoes maturation and holoblastic cleavage and develops in situ into a blastula. The first three cleavages are vertical and produce a disc of eight pyramidal cells or blastomeres. A horizontal cleavage then divides the blastomeres unequally, yielding eight large cells macromeres produce the future epidermis and eight small cells micromeres give rise to future choanocytes.
At the 16 cell stage the embryo lies just beneath the maternal choanocyte layer as a flattened disc-shaped body.
The micromeres (small cells) increase rapidly, elongate and each acquires a flagellum on its inner end facing the blastocoel. The large cells remain undivided for some time, become rounded and granular and in their middle an opening forms that functions as a mouth to ingest adjacent maternal cells. This stage is called stomoblastula by Duboscq and Tuzet.
The stomoblastula, thus, is the blastula stage of Scypha consisting of many small, elongated and flagellated micromeres and 8 spherical granular macromeres. It bears a blastocoel which opens out by an aperture, the mouth which is formed in the macromeres.
The mouth is used for engulfing the surrounding maternal cells for nutrition. The stomoblastula undergoes a process of inversion in which the embryo turns out through its mouth, so that the flagella become directed towards the outside. Now, the embryo is called amphiblastula larva.
Amphiblastula larva:
The amphiblastula larva occurs in the development of most of the Calcarea. It is more or less oval in shape and consists of one-half of small, narrow flagellated cells and the other half large rounded granular cells. The amphiblastula larva forces its way into the adjacent radial canal and escapes through the osculum of the parent to swim for some hours with the flagellated cells directed forward.
After swimming for a few hours gastrulation takes place by the invagination of the flagellated cells. Now the larva becomes a typical gastrula with a blastopore at the invaginated side.
Post-embryonic development (metamorphosis):
The gastrula soon gets attached to some substratum like rock, sea weed, etc., by its blastoporal end and develops into a cylinder.
At the free end of the cylinder an opening, the osculum is formed. A large number of small perforations are formed on the cylinder which are the ostia. The non-flagellated granular cells give rise to the dermal epithelium, scleroblasts and porocytes. The flagellated cells give rise to the choanocytes, archaecocytes and amoebocytes. The mesenchyme arises from both the layers.
The young Scypha now reaches the olynthus stage (Fig. 26.13) resembling the asconoid type of sponge. The adult or syconoid stage develops from the olynthus stage by pushing out of the wall, first at the middle, into radial canals. The choanocytes are shifted in these radial canals and the body wall increases in thickness. Thus, the adult Scypha is formed and its colony develops later on by further branching.
