Date & Time : 27, January 2021 11:18
General Characteristics of Porifera
- Poriferans are commonly called sponges.
- These are multicellular organisms which are sessile/sedentary in nature.
- Most of them are marine while a few are fresh water forms.
- Body is cylindrical, asymmetrical or has radial symmetry.
- Body wall is diploblastic which consists of an outer layer called pinacoderm and an inner layer called choanoderm.
- The middle layer is called mesenchyme which has various kinds of wandering amoebocytes.
- Body wall contains numerous pores called ostia through which water enters in the body through a canal system into the central body cavity, called spongocoel.
- The canal syytem mainky comprise of incurrent and excurrent canals which are lined with pinacocytes; and radial canals which are lined with choanocytes (flagellated cells).
- Body also contains one or more openings called oscula (-um/singular) through which water passes out from the body.
- Sponges feed on minute organisms and small organic particles which enter the body through water current and are ingested by the choanocytes.
- Respiration takes place by diffusion of oxygen from water the flowing into the body.
- Sponges have an exoskeleton which is made up of either spongin fibers or calcareous/siliceous spicules or a combination of both.
- Excretory product, mainly ammonia, is released from the body through outgoing water current.
- Reproduction may be asexual or sexual.
- Asexual reproduction takes place by external or internal budding (formation of gemmules) or by disintegration of body into reduction bodies.
- Sexual reproduction involves internal fertilization. Male gametes enter the sponge body through water current and fertilize with ovum with the help of choanocytes.
- The larval stage of calcaroeus sponges is amphibalstula while that of demospongiae is rhagon larva.
Classification of Porifera
Porifera is divided into three classes based on the skeleton they possess.
- Skeleton consists mainly of calcareous spicules
Examples: Sycon, Leucosolenia
2. Hexactinellida (Hyalospongiae)
- Skeleton consists mainly of siliceous spicules
Examples: Euplectella, Hyalonema
- Skeleton consists mainly of spongin fibres which may be in combination with spicules
Spicules of Sponges
- Spicules are formed by scleroblast cells present in the mesenchyme.
- They consists of an axis of organic material around which calcium carbonate or silica is deposited. Thus they are calcareous or siliceous in nature.
- They are of various shapes. Based on their main axis and number of rays, they can be classified into following types-
- Monaxons - single axis growing in one or more directions.
- Triaxons - three axes crossing each other and have six rays.
- Tetraxons - four rays.
- Polyaxons - many axes and several rays radiating from a single point.
- Spheres - spherical in shape with concentric growth.
- Desmas - formed by deposition of successive layers of silica on an ordinary spicule.
- Spicules form the skeleton of body. They thus support and protect the body.
- Smaller flesh spicules found throughout the mesenchyme are called Microscleres. They are of two types - spires (spiral in shape) and asters (star-shaped).
- Large-sized spicules are called Macroscleres.
Gemmules of Sponges
- Gemmules are asexual reproductive bodies of most of the freshwater and a few marine sponges.
- These are internal buds and are response to the hostile environment.
- These are small, hard, ball-like structure consisting of an outer capsule and an inner mass of archaeocytes.
- Capsule is made of two chitinous layers. Outer chitinous layer contains amphidisc spicules to strengthen the capsule.
- Arcaheocytes are totipotent cells and are filled with reserve food material formed by trophocyte cells.
- At one end of the gemmules an opening called micropyle is present.
- These are resistant to desiccation (drying out), freezing, and anoxia (lack of oxygen) and can lie around for long periods of time.
- Under favourable conditions, the mass of archaeocytes come out in water through micropyle and develops into a young sponge.
A Few Examples