Phoronis: Habitat, Structure and Development (With Diagram)

Habit and Habitat of Phoronida:

Phoronis is exclusively marine and is found sparingly over a wide geographical range. It lives in sandy bottom in shallow seas. In the adult stage, it is sedentary becomes enclosed by a membranous or leathery tube, within which the animal is capable of being retracted. It is a ciliary mucous feeder obtains food in the form of small, marine creatures by way of tentacular ciliary Currents.

Tube of Phoronida:

All phoronids occupy a cylindrical tube of their secretion in tentacular fold which they can move freely. These tubes usually occur in aggregations that in some species result from asexual reproduction. It is not definitely known whether the secretion comes from the entire surface of the worm or from some part thereof when freshly formed.

The secretion is fluid, transparent and sticky but on contact with water sets into a firm condition.

The secreted part of the tube is chitinous. During the sticky phase there adhere to the secretion various objects of the environment as sand grains, minute pebbles, sponge spicules, shell fragments that give the tube its definite appearance although often a short length of the clear tube free of adhered objects remains at the top.

External Structures of Phoronida:

The body of Phoronis is cylindrical, elongated and un-segmented. It is colourless and transparent but sometimes yellowish or greenish. The body is differentiated into an anterior lophophore and a posterior trunk.

(i) Lophophore:

The lophophore is horse-shoe-shaped tentacular crown lying at the anterior oral end. It consists of two prominent ridges, outer and inner, between which is a groove leading into the median mouth. There are numerous slender hollow ciliated tentacles present on the lophophore.

The tentacles are arranged in two rows, of which the one on the inner side of the lophophore is incomplete in the middle line. The tentacles are non-retractile, supported by mesodermal exoskeleton and serve to catch the food particles.

The tentacles of each row are basally fused to form a membrane continuous with the lophophoral ridge bordering the mouth. Overhanging the mouth a broad lobe called the upper lip or epistome is present. Both mouth and anus are situated at the tentacular extremity of the body separated from one another by only a short space. Near the anus open two ciliated nephridial tubes.

 

(ii) Trunk:

The trunk region of the body is narrow, slender, cylindrical and is devoid of appendages. The trunk is demarcated from the lophophore by a slight groove. The trunk is of uniform diameter throughout the length except the posterior end which becomes enlarged to form the end bulb. The trunk exhibits faint annulations.

Body Wall of Phoronida:

The body wall consists of cuticle, epidermis, basement membrane, muscular layers and coelomic epithelium. The cuticle is well-developed in the region of the lophophore. The epidermis is composed of columnar cells, neurosensory cells and gland cells. In the tentacles the epidermis is ciliated.

At the basal part of the epidermis throughout the body, lies the nervous layer. The basement membrane is placed between the epidermis and the muscular layer. The muscular layer is composed of a thin outer circular muscles and thick inner longitudinal muscles. The coelomic epithelium is syncytial in nature.

Coelom of Phoronida:

The coelom is spacious. The coelom is divided into two unequal parts by a partition or mesentery which runs across just behind the tentacles. The anterior part of coelom includes the cavities in the tentacular epistome and lophophore. This part of coelom is devoid of external openings. The posterior trunk coelom (metacoel) is quite extensive and occupies the whole length of the trunk.

The metacoel, in the adult, is usually divided into four longitudinal compartments by mesenteries, two dorso-lateral and two ventro-lateral. The posterior trunk coelom is communicated with the exterior through the nephridia. The coelomic fluid is a colourless, albuminous fluid and contains coelomocytes, red blood corpuscles, spindle bodies and pigment cells. The coelom is lined with a coelomic epithelium.

Digestive System of Phoronida:

The alimentary canal is a long tube which is bent on itself to form a U-shaped loop. It is differentiated into oesophageal, gastric and intestinal regions. The crescentic mouth receives ciliated grooves from the lophophore. The mouth leads into the oesophagus. The walls of oesophagus are thick and contain internal folds.

The oesophagus proceeds into a long descending tube called the prestomach or proventriculus. The prestomach is devoid of muscular layer but pos­sesses an internal mid-dorsal ciliated band. In the end bulb the prestomach is dilated to form a roundish stomach.

The stomach has an inner ciliated epithelium in the middle region, but at the two sides the epithelium becomes syncytial. A constriction marks the stomach from the ascending intestine which is differentiated into proximal wider region and distal narrow region. The intestine passes into the rectum and opens to the exterior through the anus.

Circulatory System of Phoronida:

The blood vessels are closed tubes with contractile walls. These blood vessels sub-serve the functions of heart. There are two main longitudinal blood vessels running along the trunk. A dorso-median or afferent vessel lies between the two limbs of alimentary canal. Anteriorly it forms an afferent ring vessel in the region of lophophore giving off one branch to each tentacle.

The ventro-lateral or efferent vessel runs along the left side of the oesophagus. Anteriorly it bifurcates before joining the efferent ring vessel of the lophophore. Blood flows in it posteriorly. Aborally, the dorso-median and ventro­lateral vessels communicate with the haemal plexus of the stomach wall. The blood contains only red blood corpuscles suspended in a colourless blood plasma.

Excretory System of Phoronida:

The excretory system consists of a pair of metanephridia which also act as gonoducts. Each nephridium is a U-shaped tube lined throughout its length by the ciliated epithelium. It opens to the exterior through nephridiopore on the side of the anus.

Nervous System of Phoronida:

The nervous system lies immediately below the cells of the epidermis. The nervous system consists of nerve fibres and nerve cells which form a distinct nervous layer beneath the epidermis.

This nervous layer becomes differentiated into nerve ring at the anterior end of the body which is regarded as the pre-oral nervous field and it gives nerves to the tentacles. The nerve ring becomes thickened and broadened mid-dorsally. In addition a lateral nerve is present. The lateral nerve innervates the trunk musculature.

Sense Organs of Phoronida:

There are no organs of special sense except the neurosensory cells in the epidermis. A pair of ciliated pits called lophophoral organs may be present dorsally in the concavity of the lophophore. They are considered glandular by some and sensory by others.

Reproductive System of Phoronida:

Phoronis is hermaphrodite, although some species are dioecious. Specialised cells on the inner coelomic wall produce sperms and ova. The gonads are loose indefinite masses located posteriorly. Usually the testis occurs to the ventral side and ovary to the dorsal side of the lateral vessel.

The mature cells are released in the body cavity and pass out through the nephridia to reach the concavity of the lophophore enclosed by the tentacles. Fertilisation occurs either in the body cavity or in the lophophoral cavity where reproductive cells are brought out through the nephridiopores.

Development of Phoronida:

Fertilised ova undergo earlier stages of development while attached to the tentacles.

The lophophoral cavity serves as a brood pouch containing ova and embryos in all the stages of development. The development is indirect. Cleavage is complete (holoblastic) and slightly unequal. The cleavage results into the formation of coeloblastula. The vagetal pole of the coeloblastula becomes flattened and invagination starts in typical embolic fashion.

The mesoderm is budded off from the endoderm. The coelomic spaces arise as diverticula from the enteric wall. After the completion of development, a ciliated larva is produced. The larva is a free swimming form and has an oval body. The preoral lobe becomes expanded and bent forward.

The alimentary canal is differentiated into oesophagus, stomach and intestine.

As the larva develops, the cilia becomes restricted to certain areas of the body. The full developed larva is known as actinotrocha larva. It is free swimming, planktonic and has a gelatinous transparent body. It is elongated ranging from 1-5 mm in length. Overhanging the mouth of the larva there is a hood-like hollow pre-oral lobe and a circlet of ciliated tentacles.

An apical plate with eye spots is present in the pre-oral lobe of the larva. A pair of excretory organs with solenocytes is present.

There is present another post-oral ciliated ridge below the mouth. A girdle of slender tentacles arising from the pre-oral lobe surrounds the anterior part of the larva. Actinotrocha larva settles down to the bottom and transforms into an adult. This larva bears close resemblance with the tornaria larva of Balanoglossus.

Affinities of Phoronida:

The affinities of Phoronida have long been a subject for speculation and difference of opinion. Due to the peculiar anatomical organisation and structural resemblances with other groups, Phoronida has been a controversial issue since its discovery. Brachiopoda and Ectoprocta resemble very closely with the Phoronida.

The Phoronida, Brachiopoda and Ectoprocta are collectively called the lophophorate coelomates. Hyman (1959) has suggested the term lophophorate for these groups because of the presence of lophophore.

Affinities with Brachiopoda:

The Phoronida shows close resemblances with Brachiopoda.

The resemblances are:

(1) Presence of horse-shoe-shaped lophophore.

(2) Presence of an epistome.

(3) Presence of U-shaped alimentary canal.

(4) Presence of a coelomic septum separating the mesosome and metasome, although poorly developed in Brachiopoda except Crania where the septum is complete.

(5) Presence of sub-epidermal nerve plexus forming a nerve centre in the mesocoel.

(6) Presence of a pair of metanephridia.

(7) The mouth originates directly from the blastopore in both.

(8) Dorsal surface between mouth and anus is extraordinarily shortened in both the cases. The Phoronida and Brachiopoda, though possess many similarities, have many structural differences which do not support the affinities of Phoronida with Brachiopoda.

The main differences are as follows:

(1) The nerve centre is supra-enteric in Phoronida, while sub-enteric in Brachiopoda.

(2) In Phoronida two sets of tentacles are present, one is the larval set and the other is the definitive set, while in Brachiopoda larval tentacles are wanting.

(3) The Brachiopoda shell cannot be correlated with the exoskeleton of Phoronida.

(4) The chitionous setae in the larval and adult Brachiopoda have no counterparts in Phoronida.

(5) The circulatory system of Phoronida is more developed than the Brachiopoda. In phoronids the blood vascular system is closed type, while in brachiopods it is open type.

(6) In Phoronida, the cleavage pattern is spiral, while in Brachiopoda the cleavage is not spiral.

Affinities with Ectoprocta:

Caldwell (1882) emphasised the affinities of Phoronida with Ectoprocta.

The idea was based on the existence of the following similarities:

(1) Same body and coelomic regionation with a definite septum between mesocoel and metacoel.

(2) Presence of horse-shoe-shaped lophophore.

(3) Presence of an epistome.

(4) Presence of U-shaped alimentary canal.

(5) The nerve centre is situated in the mesocoel and supraenteric in both. But a detailed study of two groups shows many structural dissimilarities.

They differ widely from both anatomical and embryological point of view.

The differences are:

(1) The origin of coelom is different. In Phoronida the coelom is endomesodermal in origin, while in Ectoprocta it is ectomesodermal.

(2) The region between the mouth and anus is dorsal in Phoronida but in Ectoprocta it is ventral.

(3) Circulatory system and nephridia are absent in Ectoprocta but in Phoronida both the systems are present and well-formed.

(4) The developmental sequences vary quite greatly.

Because of the presence of wide structural differences, the relationship between Ectoprocta and Phoronida cannot be justified.

Of the three lophophorate coelomates, the phoronids are nearer to the lophophorate ancestor because they resemble each other in the following points:

(1) Muscular vermiform body with crescentic lophophore.

(2) Existence of a septum between an anterior lophophore bearing part and posterior trunk.

(3) Circulatory system is of closed type with dorsal and ventral vessels.

(4) Trochophore type of larva has protonephridia.

Affinities with Annelida:

The affinities of Phoronida with Annelida are mainly based on the larval similarities. Because of their resemblances the Actinotrocha is regarded by many authors to be modified Trochophore larva.

The similarities are as follows:

(1) The tentaculate lophophore of Phoronida corresponds to the tentacular crown of Sipunculus.

(2) In both Phoronida and Annelida the mature germ cells pass out through the nephridia.

(3) The Actinotrocha larva and the Trochophore larva possess many common features.

(i) Both of them are free swimming ciliated pelagic forms with distinct pre-oral lobe,

(ii) The girdle of larval tentacles develops from the ciliary band. The cilia bordering the pre-oral lobe of Actinotrocha represent the metatroch and prototroch of Trochophore respectively. The disposition of telotroch is similar in both.

(iii) A thickening of the ectoderm of the pre-oral lobe in Actinotrocha represents the apical plate of Trochophore.

(iv) A pair of solenocytic nephridia is present.

(v) In both the larval forms the alimentary canal is similarly placed and has similar divisions. But closer examination reveals that the Annelida and Phoronida are fundamentally different in their organisation.

The most important point lies in the development of mesoderm. In Actinotrocha the mesoderm is endomesodermal and occurs as mesenchyme.

The body of Phoronida is unsegmented, but in Annelida segmentation is the main criterion in their organisation. In Trochophore mesoderm is arranged in teloblastic bands. The affinities of Phoronida and Annelida cannot be justified critically, but the larval similarities are quite striking. These larval similarities may be due to their adaptive convergence.

Affinities with Hemichordata:

Masterman (1897) tried to establish the affinities between Phoronida and Hemichordata on the grounds of similarities in the larval forms between the Actinotrocha larva of Phoronida and Tornaria larva of Hemichordata.

The similarities are as follows:

(1) The division of body of Hemichordata (proboscis, collar and trunk) corresponds to the body division of Phoronida (epistome, mesosome and trunk or metasome).

(2) A pair of glandular pockets opening into the proximal end of the stomach of Phoronida is supposed by Masterman to be the paired notochord.

(3) Presence of a septum between the middle and posterior sectors of the body in both the forms.

(4) The position of tentaculate lophophore is similar to the tentaculate arms of Cephalodiscus, an example of Hemichordata.

(5) Presence of superficial similarities in the disposition of coelom in the larval forms.

But the thorough study reveals the following differences:

(1) The three divisions of the body of Phoronida are not justified by embryological studies. As a matter of fact, Phoronida has two divisions in the body. The epistome of Phoronida is not the body region and does not contain coelom like that of proboscis of Hemichordata.

(2) The mesocoel of Phoronida communicates to the exterior through the metanephridia which are absent in Hemichordata.

(3) The coelom in Actinotrocha is divided in three compartments, while in Trochophore, the collar coelom and trunk coelom are paired.

(4) The notochordal nature of glandular pockets in Phoronida is difficult to establish. From the above discussion it is quite apparent that most of the arguments forwarded by Masterman to establish the affinities between the Phoronida and Hemichordata are not supported by embryological facts, therefore, the affinities between these groups cannot be established.

Members of Protostomia (Platyhelminthes, Nemertines, Sipunculoidea, Echiuroidea, Annelida and Mollusca, etc.) appear to be related to one another as they show spiral cleavage, formation of mouth at the site of or as a part of blastopore, endomesodermal origin of coelom and free-swimming larvae that are Trochophores.

On the other hand Deuterostomia (Hemichordata, Echinodermata, etc.) have body divisions into three regions, blastopore becoming anus, enterocoelous mode of origin of coelom and larva called dipleurula.

Because of its dualistic affinities with the Protostomous phyla on one hand and the Deuterostomous phyla on the other, it appears that Phoronida forms a sort of connecting link between Protostomous phyla and Deuterostomous phyla.