9.3 Invertebrates
Most biologists have divided the animals into two groups based on the presence or absence of a vertebral column (backbone).
The group invertebrates lack vertebral columns while the group vertebrates possess vertebral columns. Invertebrates account for 95% of known animal species, invertebrates are divided into eight major phyla, which are Porifera, Coelenterata, Platyhelminthes nematodes, annelids, Mollusca, Arthropoda, and Echinodermata.
9.3.1 Phylum Porifera
(Greek: Poros means channels, Latin: Ferre means The name Porifera was given by Robert E. Grant in 1836. Phylum Porifera is also known as sponges and possesses the following characteristics.
Habitat:
Their larval stage is motile while the adult is sessile (attached to submerged rock), about 10,000 species have been recorded. Their size ranges from a few millimeters wide to more than a meter long (e.g., scolymastra Roubini from Antarctica).
Body:
Poriferans are the simplest multicellular animals having no tissues and organ organization. Asymmetrical or radially symmetrical. The body wall is formed of an outer dermal layer known as pinacoderm, which contains pinacocytes, and an inner layer, the choanoderm, which contains flagellated collar cells known as choanocytes. In between these two terms is a gelatinous matrix, the mesenchyma containing many kinds of wondering cells.
They have a single body cavity, the spongocoel, divided into canals. The body contains numerous incoming or incurrent pores, the Ostia, and a single large e excurrent pore, the osculum.
Nutrition: Sponges are heterotrophic and depend on food coming to their body with water current through ostia and moved in by flagella of choanocytes, where intracellular digestion takes place, and the digested foods are transported by mesenchymal cells. The waste material either diffuse out of the sponges directly through the body wall or through the osculum.
The nervous system is absent, but neuron and neurosensory cells help to coordinate water flow. The skeleton is formed of needle-like structures the spicules (either calcareous or silicious) while some sponges, like bath sponges, possess sponging fibers, and osculum, and Ostia also contain spicules. The sponges possess both asexual and sexual reproduction.
Asexual, either by external budding or internal budding (gemmule formation), regeneration is also common. Sponges are mostly hermaphrodite (Bisexual) and protandrous (male gametes develop before the female). Fertilization is internal and occurs in mesenchyma. The sponges have probably evolved from flagellated protists known as choanoflagellates.
Examples:
sycon (marine sponge), Leucosolenia (erect tube shaped), Euplectella (glassy framework), and Spongilla (freshwater sponge).
Evolutionary adaptation in Porifera:
Sponges have intracellular digestion, which is facilitated by flagellated collar cells. Respiration occurs by diffusion because almost each body cell is in contact with water.
Detritus is dead and desires organic Transportation of substances (food, excretory products) either by diffusion or by water current through canal systems. "Water enters through ostia into spongocoel and goes out of the body through the osculum." They have weak coordination, only some neurosensory cells and neurons coordinate water flow.
Importance of sponges:
The bath sponges are used for washing and bathing by humans. Sponges can greatly absorb water and thus are used in surgical operations for absorbing fluid and blood. In large buildings, sponges are used for sound absorption, to prevent echo. They are also used for decorative purposes and used in shoe or vehicle polishing.
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