Phylum Porifera

Date & Time : 16, November 2024 05:20

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.

1. Calcarea

Skeleton consists mainly of calcareous spicules

Examples: Sycon, Leucosolenia

2. Hexactinellida (Hyalospongiae)

Skeleton consists mainly of siliceous spicules

Examples: Euplectella, Hyalonema

3. Demospongiae

Skeleton consists mainly of spongin fibres which may be in combination with spicules

Example: Spongilla

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.