Body structure [Phylum Parazoa]
Probably the best way to describe the body of a sponge is to define its various parts: The skin or ectosome is the outer layer or layers of cells; usually this is a single or double-layer but in a few species it takes the form of a thick cortex made up largely of muscle fibres. The pores are very minute openings in the skin through which water is drawn into the sponge, and in this way, they form the inhalant current.
The vents are rather larger openings in the skin through which the exhalant current of water is passed out of the sponge. As previously explained, these are usually called oscula, a word which tends to mislead as to their function.
The canals are tubes or channels running through the body along which water flows continuously, except when the sponge is quiescent. It is drawn in through the pores and driven out forcibly through the vents.
The flagellated chambers are typically spherical or thimble-shaped chambers into which the canals open, and from which run similar canals. Those leading into a chamber are called inhalants, and those leading out are called exhalants.
The exhalant canals in most sponges soon join up with neighbouring exhalant canals to form larger canals and so on progressively until a single large exhalant canal (cloaca) carries water to the vent. The flagellated chambers are so-called because they are lined with cells, each of which bears a flagellum surrounded by a protoplasmic collar. (Although this definition holds for the majority of sponges, in the glass sponges (Hexactinellida) the flagellated chambers are large and thimble-shaped and are not equipped with inhalant or exhalant canals.
By contrast, in the so-called ascon sponges (Leucosolenia) the whole central cavity, the spongocoel, is lined with collared cells. It receives the inhalant current directly through pores, and communicates directly with the exterior through a vent.)
The collared cells or choanocytes line the flagellated chambers.
Food of Sponges
Each collared cell has a spherical or columnar body bearing at its free end a protoplasmic funnel-like collar, from the bottom centre of which springs a protoplasmic whip or flagellum. As the flagellum lashes, a vortes is created around the collar, and the result of the concerted action of thousands of collared cells is to suck water in through the pores, drive it through the canals and forcibly eject it through the vents The inhalant current brings in food and oxygen and the exhalant current carries away the waste products of digestion and respiration.
Collared cells are very small and their structure in life was for a long time impossible to determine satisfactorily, even using the higher powers of ordinary microscopes. Consequently, their collars were thought to be made up of thin sheets of the protoplasm of uniform consistency.
With improved microscopes, they have been shown to have a fine structure which includes numerous longitudinal fibrillae. Since the choanocytes form the most conspicuous feature of the sponge body, this is sometimes regarded as consisting of an ectosome, or skin, and a choanosome comprising everything internal to it. Food obtained by the collared cells is passed on to wandering cells or amoebocytes which transport it to other parts of the body.
Star-shaped or stellate cells form a loose network filling the spaces between the labyrinth formed by the inhalant and exhalant canals. (Alternatively, we can regard the body of a sponge as filled with a network of star-shaped cells through which the canals run.)
The amoebocytes are cells which wander about in the spaces between the star-shaped cells. They are called amoebocytes because when active they are continually changing shape, like the protozoan animal Amoeba. There are various kinds of amoeba cytes, some carry oil globules and other nutrients, while others build the skeleton or have functions
that are not, as yet, fully understood. The skeleton and the various reproductive cells will be discussed in detail below.
Sponges have no special organs of locomotion.
and are mainly sedentary, although a small amount of creeping movement can take place, especially in the post-larval stages, and in some species even in the adult. They have no special sense-organs, although a few species show some sensitivity to light and touch, how far this is dependent upon the possible presence of rudimentary nerve cells or is a property of all the cells in the body has yet to be determined.
The food of sponges is believed to be finely divided. organic particles or bacteria, or both, and there is evidence that freshwater sponges feed to some extent on their symbiotic algae. The normal food must clearly be of minute proportions, otherwise, it would not enter the microscopic pores in the ectosome. Furthermore, sponges grow to unusually large sizes in water contaminated with decaying matter, and rich in organic particles and bacteria.
Some sponges, especially those with large vents and deep tubular cloaca, contain numerous commensals. These may number thousands for a single sponge and include a variety of small crustaceans and polychaete annelid worms. In contrast, sponges have few predators. A few are eaten by nudibranch or small gastropod molluscs, but almost as fast as these eat the wounds heal.