Quiz 12: Sponges and Placozoans
Yes, there are single-celled animals. These are also referred to as protozoans. They are unicellular eukaryotic living organisms. Hence, a diverse group of single-celled animals exist on Earth. The unicellular or single-celled eukaryotes are classified into following four classes depending upon their mode of locomotion: • Flagellates • Amoeboid • Sporozoans • Ciliates Hence, phylum protozoa are divided into four classes.
The characteristics of sponges are: • They are Multicellular animals. Their body is an aggregation of several types of cells differentiated for various functions, some of which are organized into incipient tissues of a low level of integration. • Body has pores (ostia), canals, and chambers that form a unique system of water currents on which sponges depend for food and oxygen. • All sponges are mostly marine or totally aquatic. • They exhibit radial symmetry or no symmetry. • All adults are sessile and attached to the substratum. • No organs or true tissues present, digestion occurs intracellularly, excretion and respiration occurs by diffusion. • Asexual reproduction occurs by buds or Gemmules and sexual reproduction by eggs and sperms; free-swimming flagellated larvae seen in most species. • Outer surface consists of flat pinacocytes; most interior surfaces are lined with flagellated collar cells (choanocytes) that create water currents; a gelatinous protein matrix called mesohyl contains amebocytes of various types and skeletal elements.
Asconoid type of canal system - They have the simplest organization. Water is drawn into the sponge through microscopic dermal pores by beating of flagella on choanocytes. These choanocytes line the cavity called Spongocoel. The choanocytes filter the water and extract food particles from it and the used water is expelled through a single large osculum. They are usually small and tube-shaped. This system is found only in class Calcarea. E.g. - Leucosolenia Syconoid type of canal system - They have a tubular body and single osculum and the body wall lining the Spongocoel is thicker and more complex than Asconoid types. The lining is folded outwards to make choanocytes - lined canals. This increases the surface area of the wall covered by choanocytes. Water enters the system through dermal ostia which lead to incurrent canals. The water then filters through tiny pores called prosopyles into radial canals. In the radial canals the food is ingested by the choanocytes. The flagella of the choanocytes force the used water through pores called apopyles into the cavity called the Spongocoel. The water then exits the body from the body through the osculum. This type of system is found in members of class Calcarea and in some members of class Hexactinellida. Leuconoid type of canal system - It is the most complex in organization. The surface area of the food-collecting regions with choanocytes is greatly increased as choanocytes line the walls of small chambers where they can effectively filter all the water present. The sponge body is composed of an enormous number of these tiny chambers. Clusters of chambers lined with flagellated choanocytes are filled with water from incurrent canals. The used water is then discharged into excurrent canals that eventually lead to an osculum. They are seen in the class Calcarea. E. g. - Leuconia