INTRODUCTION :
Phylum porifera includes about 5000 species. Poriferans are pore bearing first multicellular animals. The pores are known as ostia.
Phylum porifera are the lowest multicellular animals belonging to the kingdom Animalia. The word "Porifera" mainly refers to the pore bearers or pore bearing species. Based on the embryological studies , sponges are proved as animals and are classified into a separate phylum in the animal kingdom.
The poriferans have a spongy appearance and are therefore called sponges. They are attached to the substratum and do not move. They have the ability to absorb and withhold fluids. They were initially regarded as plants due to the green colour and their symbiotoc relationship with algae. Later, their life cycle and feeding system were discovered and they were include in the animal kingdom.
CHARACTERISTICS OF PHYLUM PORIFERA
1) Porifera are all aquatic, mostly marine except one family Spongillidae which lives in freshwater.
2) They are sessile and sedentary and grow like plants.
3) The body shape is vase or cylinder-like, asymmetrical or radially symmetrical.
4) The body surface is perforated by numerous pores, the Ostia through which water enters the body and one or more large openings, the oscula by which the water exists.
5) The multicellular organism with the cellular level of body organization. No distinct tissues or organs.
6) They consist of outer ectoderm and inner endoderm with an intermediate layer of mesenchyme, therefore, diploblastic.
7) The interior space of the body is either hollow or permeated by numerous canals lined with choanocytes. The interior space of the sponge body is called spongocoel.
8) Characteristic skeleton consisting of either fine flexible spongin fibers, siliceous spicules or calcareous spicules.
9) Mouth absent, digestion intracellular.
10) Excretory and respiratory organs are absent.
11) Contractile vacuoles are present in some freshwater forms.
12) The nervous and sensory cells are probably not differentiated.
13) The primitive nervous system of neuron arranged in a definite network of bipolar or multipolar cells in some, but is of doubtful status.
14) The sponges are monoecious.
15) Reproduction occurs by both sexual and asexual methods.
16) Asexual reproduction occurs by buds and gemmules.
17) The sponge possesses a high power of regeneration.
18) Sexual reproduction occurs via ova and sperms.
19) All sponges are hermaphrodite.
20) Fertilization is internal but cross-fertilization can occur.
21) Cleavage holoblastic.
22) Development is indirect through a free-swimming ciliated larva called amphiblastula or parenchymula.
23) The organization of sponges are grouped into three types which are ascon type, sycon type, and leuconoid type, due to simple and complex forms.
• Examples: Clathrina, Sycon, Grantia, Euplectella, Hyalonema, Oscarella, Plakina, Thenea, Cliona, Halichondria, Cladorhiza, Spongilla, Euspondia, etc.
CLASSIFICATION OF PHYLUM PORIFERA
Based on the type of skeleton system the phylum Porifera is divided into three classes :
Class 1: Calcarea or Calcispongiae
Class 2: Hexactinellida or Hyalospongiae.
Class 3: Demospongiae
1) Calcarea : (calcarius: lime / calcium) :
Habitat: Exclusively marine
Habit: Solitary or colonial nature.
Endoskeleton: calcareous spicules composed of calcium carbonate
Symmetry: Radially symmetry
Shape: Cylindrical shape
Examples: Sycon, Leucosolenia
2) Hexactinellida : (Hex: six, actin: ray, idea: terminal)
Habitat: Exclusively marine (deep sea)
Habit: Solitary in nature.
Endoskeleton: six- rayed siliceous spicules. Symmetry: Radially symmetry Shape: Cylindrical shape. Examples: Euplectella, Hyalonemma etc.
3) Demospongiae :
Habitat: Mostly marine and some are freshwater
Endoskeleton: Siliceous spicules or sponging fibres or both or none The spicules are monaxon or tetraxon but never six-rayed
Symmetry: asymmetrical.
Shape: Irregular Canal system complicated. Spongocoeal is totally absent.
Examples: Spongilla.
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