Sponges were earlier thought to be plants because they are sedentary and grow asymmetrically by budding and branching. In fact bath sponges are cultivated like plants by planting their cuttings on rocks and soil in the Mediterranean region.
Sponges also show no response to touching or other stimuli and have no apparent food catching organs as do other animals. But Aristotle (384-322 BC) recognized them as animals. In his book Systema Naturae, the Swedish naturalist, Carolus Linnaeus (1758) classified sponges in Phylum Coelenterata on the basis of these being sedentary, diploblastic and having spongocoel that is comparable with coelenteron and life cycle having a free swimming ciliated amphiblastula larva which is similar to planula of coelenterates. But coelenterates have all other metazoan characters and hence are distinct from sponges.
Robert Grant (1836) established a separate group for sponges and named it Porifera, based on his observation that water flows in their bodies through minute pores.
Protozoan characters of Porifera. The protozoan characters in sponges include cellular independence in their bodies as in colonial protozoa; totipotent nature of their cells; absence of body organs or tissue level organisation; absence of gonads and digestion being intracellular by endophagy. They also have collar cells and amoeboid cells as found in colonial choanoflagellates.
Metazoan characters of Porifera. Sponges are multicellular animals and diploblastic acoelomates with a non-cellular mesogloea between ectoderm and endoderm as found in coelenterates. Sponges reproduce by budding and branching like cnidarians. Their reproduction is metazoan type with freely swimming, ciliated amphiblastula, stomoblastula and parenchymula larvae. The zygote develops by cleavage giving off blastula and gastrula stages. They possess a water vascular system which jelly fishes also have and spongocoel can be compared with coelenteron of cnidarians.
GROUPS RELATED TO PORIFERA
Mesozoans were described by van Beneden (1876) who considered them to be missing link between Protozoa and Metazoa. They are ciliated, worm-like multicellular animals, which are parasitic on marine invertebrates such as Platyhelminthes, annelids, molluscs and echinoderms. They are very tiny, 0.5-7.0 mm long, having slender and vermiform body composed of only 20-30 cells that are arranged in two layers, which can be compared with ectoderm and endoderm of metazoans. They have complex trematode-like very complex life cycle with a ciliated larva. They are considered to be primitive and related to ciliates and have probably descended directly from Protozoa. However, some people consider them to be degenerated flatworms.
There are two classes in Mesozoa, namely, Rhombozoa which are parasitic in kidneys of cephalopods and Orthonectida which parasitize polychaetes, molluscs and echinoderms, for example, Rhopalura.
Phylum PLACOZOA (plax or placos=tablet)
The phylum was proposed by K.G. Grell (1971) for a single marine species, Trichoplax adhaerens, which was discovered in 1883 in a European seawater aquarium. The organism has a flat, plate-like asymmetrical body of 2-3 mm diameter that constantly changes shape. It is diploblastic with dorsal and ventral epithelia that lack a basal lamina. A gelatinous matrix forms intermediate layer which contains fibre cells, vacuoles etc. The dorsal epithelium is made of flagellated cover cells in which bright spheres are embedded. There are no organs, nerves or muscles in the body. The animal glides over food with the help of cilia and secretes digestive enzymes over it and absorbs the product by saprophagy, for which cells of the ventral epithelium possess microvilli that increase the absorptive surface.
Placozoans are closely related to poriferans and have been classified with them in an independent phylum under subkingdom Parazoa. It is believed that Mesozoa, Porifera and Placozoa have evolved from some common unicellular ancestor or colonial protozoan during early evolution and out of the three, Placozoa and Porifera are more closely related, while mesozoa owing to their parasitic mode of life have undergone excessive transformation.
Huxley (1875) and Sollas (1884) created a separate Subkingdom Parazoa for sponges and proposed that they be kept separate from other metazoans, which have a tissue grade body organization. Absence of body organs, nervous tissue, cellular differentiation and cell specialisation with cellular independence make them distinct from other metazoans. Unlike most metazoans, they lack a true mouth and possess peculiar collar cells and their digestion is intracellular.
There are two theories to explain the origin of sponges as well as other metazoans which are as follows:
1. Syncytial ciliate hypothesis postulates that poriferans originated from some multinucleated ciliates by the division of cytoplasm into as many parts as the number of nuclei. This is evidenced by the fact that sponges and their closest relatives, mesozoans and Placozoans all have multicellular body containing ciliated cells.
2. Colonial flagellate hypothesis, which appears to be more convincing, says that poriferans originated from some colonial choanoflagellate ancestor similar to Proterospongia, which had choanocytes and amoebocytes embedded in a gelatinous matrix. Later, as the colony became more complex and cells became specialised and as the animals became sessile probably due to abundance of planktonic food, the external flagellated cells migrated inside the body to line a central cavity, and gradually a complete water canal system developed. Similarities in cellular organization of Porifera, Mesozoa and Placozoa also support this hypothesis.
Sponges seem to have diverged very early in evolution from the main metazoan lineage and have not ever since evolved into any other kind of organism but maintained their distinctness and remained unchanged since Palaeozoic. They still maintain their isolated phylogenetic position and hence most workers prefer to place them in a separate subkingdom PARAZOA. Placozoans, which are represented by a single species, Trichoplax adhaerens, appear to be their closest relatives and hence are also included in Parazoa but as a separate phylum. But the phylogenetic position of mesozoans, which also display simple body organization similar to sponges, is not certain as they are parasitic animals and hence their simplicity may have been either due to their primitiveness or by degeneration from some helminth-like ancestor. Hence mesozoans are placed in a separate phylum but close to Parazoa.
ORIGIN OF METAZOA
Metazoans possess organ system of body organization in which tissues develop into organs to perform different functions in the body. Coelenterates are radially symmetrical diploblastic Metazoa while others from helminths onwards are bilaterally symmetrical and triploblastic. There are three theories to explain the origin of Metazoa.
1. Colonial theory. This theory was proposed by Butschli, Lankester & Haeckel (1874) and Hyman (1940), which says that metazoans evolved from some colonial flagellate ancestor similar to Volvox or Proterospongia. 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.
2. Syncytial theory. This theory was proposed by Hanson (1958) and Hadze (1963) and proposes that multinucleate ciliates must have been 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 and given rise to Metazoa.
3. Polyphyletic theory. The theory was proposed by Greenberg (1959) and Preston (1967) and postulates that metazoans originated from many types of protozoan ancestors in different lineages by all the methods proposed in the above theories.