Sponges were initially treated as plants because they are sedentary and grow asymmetrically by budding and branching. Moreover in Mediterranean region, bath sponges are cultivated like plants by planting their cuttings. Sponges also show no response to touching or other stimuli. They have no superficial food catching organs like other animals.
But Aristotle documented them as animals. Carolus Linnaeus, classified sponges into Phylum Coelenterata in his book ‘Systema Naturae’. Sponges are sedentary, diploblastic animals with central cavity known as spongocoel (similar to coelenteron of cnidarians). Their life cycle includes a free swimming ciliated amphiblastula larva (similar to planula larva of Coelenteron).
Robert Grant established a distinct group for sponges called as Porifera. According to him the water flows into and out of the sponge body through minute pores.
Protozoan characters of Porifera
Metazoan characters of Porifera
Phylum Mesozoa: Van Beneden considered Mesozoans as missing link between Protozoa and Metazoa. Mesozoans are ciliated, worm-like multicellular animals. They live as parasites on marine invertebrates like Platyhelminthes, annelids, molluscs and echinoderms. They are very tiny animals with slender, vermiform body composed of 20-30 cells arranged in two layers (comparable to ectoderm and endoderm of metazoans). They have complex trematode-like life cycle with a ciliated larva. They are considered to be probably descended directly from Protozoa and are related to ciliates.
There are two classes in Mesozoa,
Phylum Placozoa: Phylum placozoa was suggested by K.G. Grell to include a single marine species, Trichoplax adhaerens. Trichoplax adhaerens was discovered in a seawater aquarium in Europe. This organism has a flat, plate-like asymmetrical body which constantly changes shape. It is diploblastic in nature. A gelatinous matrix acts as intermediate layer. This layer consists of fiber cells and vacuoles. There are no organs, nerves or muscles in its body. This animal slides over food with the help of cilia. Mode of nutrition is saprophagy; it secretes digestive enzymes over its food and absorbs the product. It possesses microvilli to increase the absorptive surface.
Placozoans are closely related to poriferans and are classified as an independent phylum under subkingdom Parazoa. It is supposed that during early stages evolution Mesozoa, Porifera and placozoa must have evolved from a common unicellular ancestor. Out of these three, Placozoa and Porifera are more closely related, while mesozoa has undergone tremendous transformation due to its parasitic mode of life.
Huxley and Sollas created a separate Subkingdom Parazoa to include sponges. They kept parazoans separate from other metazoans. Parazoans are different from metazoans due to the absence of body organs, nervous tissue, cellular differentiation and cell specialization. Parazoans also lack a true mouth and possess peculiar collar cells. The digestion in parazoans is intracellular.
The origin of sponges and other metazoans can be well explained with the help of following two theories:
Syncytial ciliate hypothesis: This hypothesis assumes that poriferans originated from multinucleated ciliates by the division of cytoplasm. This is proved by the fact that sponges and their closest relatives, mesozoans and placozoans all have multicellular bodies with ciliated cells.
Colonial flagellate hypothesis: This hypothesis assumes that poriferans originated from colonial choanoflagellate ancestors similar to Proterospongia. Proterospongia had choanocytes and amoebocytes embedded in a gelatinous matrix. Later, as the colony became more and more complex, the cells became specialised. As the animals became sessile due to availability of plenty of planktonic food, the external flagellated cells migrated inside the body to line a central cavity. Gradually a complete water canal system developed. Similarities in cellular organization of Porifera, Mesozoa and Placozoa also support this hypothesis.
Sponges have diverged early in evolution from the main metazoan lineage and have not evolved into any other kind of organism since then. Sponges have maintained their distinctness and remained unchanged since Palaeozoic. They still maintain their isolated phylogenetic position and hence it is placed in a separate subkingdom Parazoa.
Placozoans (represented by a single species, Trichoplax adhaerens), appear to be the closest relatives of sponges. Hence placozoans are also included in Parazoa but as a separate phylum. The phylogenetic position of mesozoans is not certain as they are parasitic animals. Though parasitic mesozoans are very simple may be due to their primitiveness. Hence mesozoans are placed in a separate phylum but much close to Parazoa.
Metazoans possess organ grade of body organization in which tissues develop to form organs. These organs perform various functions in the body. There are three theories which explain the origin of Metazoa.
Colonial theory: Butschli, Lankester & Haeckel and Hyman proposed this theory. According to this theory, metazoans have evolved from colonial flagellate ancestor similar to volvox. Haeckel said that the hollow flagellate spherical colony of Volvox appears like blastula or gastrula of metazoans. Some protozoans like Gonium, Synura and Pandorina form morula-like solid colonies that superficially resemble metazoans.
Syncytial theory: Hanson and Hadze propose this theory. According to this theory, multinucleate ciliates are the ancestors of metazoans. Ciliates possess high grade of organelle development and body organization among protozoans. By partitioning the multinucleated cytoplasm with cell membranes the ciliate body could have become multicellular to give rise to Metazoa.
Polyphyletic theory: Greenberg and Preston proposed this theory. According to this theory, metazoans originated from many types of protozoan ancestors in different lineages.
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