STRUCTURE, AFFINITY AND LIFE HISTORY OF PHORONIDA

INTRODUCTION

Phoronida are long and thin worm like organisms.  All phoronids reproduce sexually. Some of them are hermaphrodite while other aredioecious. Fertilization is internal as well as external. Intermediate larval stage is present. This species generally live in coastline of mostly all oceans and seas except the Antarctic ocean. Phoronids are generally algae feeder and small animal larvae and other plankton floating in the water.

This creature lives inside the slender tube formed by its own body. There is no distinct head. The mouth is covered by the flap of skin and a crown of tentacles known as lophophore. When the lophophore is extended at the top of the body cilia on the side of tentacles draws the food particles to the mouth. The unwanted material can be rejected by closing the flap above the mouth or by the tentacles by switching of the cilia in reverse.

STRUCTURE OF PHORONIDA

• Body is unsegmented and cylindrical as well as colourless and transparent. The body is divided into anterior lophophore and exterior trunk. They can be 2-20 cm long. They lack cuticle but rigid tube of chitin is secreted by them.

Trunk is a region where body is narrow, slender and cylindrical without any appendages. Trunk can be marked by lophophore by slight groove.

The bottom end of their body id ampullae which help them to anchor in the tube of chitin and retract its body really very quickly when threaten. Phoronids can move their body within tube but are unable to leave them.

Nervous system

• The nerve is located at the base of nerve lophophore just under the skin. The nerve ring supplies nerve to tentacles, body wall and muscles. Nerve ring further bifurcates into nerve trunks. Nerve trunk further has giant bulb containing axon at its end. Axon coordinates the retraction of body when danger occurs.

Movement

• They have very limited and slow movements such as partial emerging from the tube bending the body when extended and lophophore flicking into the mouth.

Digestive system

• The alimentary canal is long and u-shaped looped consist of three main region. These three regions are oesophagus, gastric and intestinal region.
• The wall of oesophagus is thick consisting of numerous folds that open into the mouth.
• Mouth posses ciliated groove of lophophore.
• Pre stomach or pre ventricular is long descending tube after oesophagus. It posses ciliated band but are devoid of muscular bands. Dilation of pre stomach leads to stomach in the end.
• The stomach has inner ciliated epithelium. There are two regions one is proximal wider and distal narrow region.
• Intestine open outside of the body through anus.

Circulation

• Heart is absent but major vessel behaves like heart. Blood contains red blood corpuses and plasma hence main component of blood is haemoglobin. Some species contain twice oxygen carrying capacity as much as of human.
• Blood vessels arrangement starts from the base of lophophore to the middle of the body. Podocytes on the wall of blood vessel perform filtration.
• There are two important blood vessels running along the trunk.
• Afferent a dorso median vessel passes between two limbs of alimentary canal.
• Efferent or ventro lateral vessels passes along the lateral side oesophagus.
• Dorso median or ventro lateral communicates with the haemal plexus of stomach.

Excretory system

• Consist of pair of protonephridia. It opens outside through the nephridiopore into the anus.

AFFINITIES OF PHORONIDA

Due to the older anatomical organization and structural resemblance with other group their classification has been difficulty since from its discovery.

Affinities with Brachiophora

Similarities

• Presence of lophophore
• Presence of epistome
• Presence of u-shaped alimentary canal
• Presence of sub epidermal nerve ring
• Presence of pair of metanephridia
• Mouth develop from the blastocoels
• Surface between mouth and anus is less in both the cases

Differences

• Nerve centres are sub-centric in Brachiopoda while supra centric in Phoronida.
• In phoronida two sets of tentacles are present one is larval set and other is definitive set while in brachipoda larval set are wanting
• Exoskeleton in both the cases differ
• In Phoronida the circulatory system is closed type while in Brachiopoda is open type.
• The cleavage in Phoronida is spiral while in other it is not spiral.

Affinities With Ectoprocta

Similarities

• Presence of lophophore
• Presence of epistome
• U-shaped alimentary canal
• Same body and coelomic regionation

Differences

• The region between mouth and anus is dorsal in Phoronida while in Brachiopoda its ventral
• Origin of coelom differ in both
• Ectoprocta lack Circulatory system and nephridia.
• Differences in developmental stages.

Affinities with Annelids

Similarities

• Tentacular lophophore is similar to tentacular crown of sipunculus.
• Mature gametes passes through the nephridia.
• Larval form of Phoronida posses similarities with Trocophore
• Both of the larvas are free swimming, pelagic and posses distinct preoral lobe.
• The girdle of larval tentacles develop from ciliary band
• Presence of pair of  solenocyte nephridia.

Differences

• The body of phoronids are unsegmented while in annelids segmentation is main criteria.
• There are many differences between phoronids and annelids but their larvas are quite similar which can be due to adopting convergence.

Affinities with Hemichordates

Similarities

• Division of body in both the cases is similar
• The presences of tentacular lophophore is similar to the tentacular arm of Cephalodiscus.
• Presence of septum between the middle and posterior sector of the body.

Differences

• Actually the phoronids have two divisions in the body.
• The notochordal region of glandular pocket in phoronida is difficult to interpret.

LIFE HISTORY OF PHORONIDA

• They generally reproduce sexually from spring to autumn. There is presence of swollen gonads around the stomach where the gametes are produced. The gametes passes from metacoelom to metanephridia and exit through the nephridiopore. While some are capture from the lophophore of individual of same species.
• In most species eggs and sperm is release and fertilization takes place in water. While in other sperms are transfer directly to the female where fertilization occur inside the female body cavity.
• Initial division of the eggs are holoblastic where the cell divides completely and radial.  The process conditional.  Mesoderm formed from early mesenchyme.
• Larva of phoronid is free swimming known as actinotroch that feeds upon planktons. Its body is gelatinous and transparent. It have anus at the bottom and fringed with the cilia on the overhang of the mouth of the larva there is hood like hollow pro oral lobe and circlet of ciliated tentacles.
• Post oral ciliated ridge below the mouth is also present.
• This larva settle down to the bottom and transformed in the adult. It bears close resemblance with tornaria larva of the Balanoglossus.

CONCLUSION

• The Phoronida, brachiopod and Ectoprocta are collectively called as lophophorates coelomates because they all posses lophophore. The Phoronida shows great resemblance with Brachiopods and Ectoprota but not that much that it should kept with them in classification. They are sometime known as horseshoe worm. They are small phylum of marine animals and build up upright tube of chitin to support and protect their soft bodies. The tube comprises 3 layered organic inner cylindrical and an agglutinated external layer.
• They lives in most ocean and seas, including Arctic ocean and excluding the Antarctic ocean, and between the intertidal zone.
• The phylum has two genera with no class and order. There are only 12 undisputed adult phoronida species. 25 different types of larva has been identified.