An Example of Phylum Phoronida: Phoronis

In this article we will discuss about Phoronis:- 1. Habit and Habitat of Phoronis 2. External Structures of Phoronis 3. Body Wall 4. Coelom 5. Digestive System 6. Circulatory System 7. Excretory System 8. Nervous System 9. Reproductive System 10. Development.

Contents:

1. Habit and Habitat of Phoronis
2. External Structures of Phoronis
3. Body Wall of Phoronis
4. Coelom of Phoronis
5. Digestive System of Phoronis
6. Circulatory System of Phoronis
7. Excretory System of Phoronis
8. Nervous System of Phoronis
9. Reproductive System of Phoronis
10. Development of Phoronis

1. Habit and Habitat of Phoronis:

Phoronis is exclusively a marine animal and lives in sandy bottom in shallow seas. In the adult stage, it is sedentary and becomes enclosed by a membranous or leathery tube, within which the animal is capable of being retracted. The tube is formed from the secre­tion of the animal.

Particles of foreign mat­ters, such as, sand grains, sponge spicules, shells adhere to the tubes (Fig. 20.1A). Phoronis ovalis has the habit of burrowing in rocks and calcareous shells. It has a wide range of geographical distribution. It is a ciliary-mucous feeder and eats on micro-organisms brought by water current.

2. External Structures of Phoronis:

Phoronis has an un-segmented, tubiculous and elongated body. The body is differenti­ated into three parts—prosome, mesosome and metasome, each containing its coelomic cavity. The prosome forms the epistome, a fold overhanging the mouth dorsally. The mesosome bears the lophophore and the metasome or trunk is slender and cylindri­cal, with a bulb-like posterior end.

The lophophore is horse-shoe-shaped with spi­rally coiled lateral cornu. The cornu are supported by mesodermal skeleton. A bunch of slender ciliated hollow tentacles is present on the lophophore. In Phoronis, the crown of tentacles is separated by a groove. Both the mouth and anus are located at the tentacular end.

Overhanging the mouth a broad flap of tissue called upper lip or epistome is present. Two ciliated nephridial tubes open near the anus. The trunk region or metasome of the body is narrow, cylindrical and is devoid of appendages. The trunk is of uniform diam­eter throughout the length except at the posterior end where it becomes enlarged to form the end bulb or ampulla. The trunk shows faint annulations.

3. Body Wall of Phoronis:

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

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

4. Coelom of Phoronis:

The coelom of Phoronis is spacious. The coelom is divided into three major parts by an oblique diaphragm which runs across posterior to the tentacular crown. The anterior part of the coelom, the protocoel which includes the cavity of tentacular epistome and the mesocoel incudes the a lophophoral coelom which lies at the base of the lophophore (tentacular crown). This part of the coelom is devoid of external openings.

The posterior trunk coelom (metacoel) is quite extensive and occupies the whole length of the trunk and ampulla. The metacoel, in adult, be­comes usually divided into four longitudinal compartments by mesenteries, two dorso­lateral 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.

5. Digestive System of Phoronis:

The alimentary canal is a long tube which bends on itself to form a ‘U’-like loop and is broadly differentiated into oesophageal, gas­tric and intestinal regions (Fig. 20.1B). The crescentic mouth receives ciliated grooves from the lophophore. The mouth leads into the oesophagus.

The wall of the oesophagus is thick and contains internal folds. The oesophagus leads into a long descending tube called pre-stomach or proventriculus. This part lacks muscular layer but possesses an internal mid-dorsal ciliated band. The pre-stomach in the end bulb becomes 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. The ascending limb of the alimentary canal is the slender intes­tine which is demarcated from the stomach by a constriction. The intestine passes into the rectum and opens to the exterior through the anus.

6. Circulatory System of Phoronis:

The phoronids have a closed blood vascular system containing only red blood corpuscles with haemoglobin. The blood vessels are closed tubes and are contractile in nature. These blood vessels sub-serve the functions of heart. There are two main longitudinal blood vessels running along the trunk (Fig. 20.1C).

These vessels are:

(a) Afferent or dorso-median vessel and

(b) Efferent or ventro-lateral vessel.

The blood flows from the posterior to the ante­rior side through the dorso-median vessel, whereas by the ventro-lateral vessels the blood is carried to the posterior side. The dorso-median vessel in the lophophoral coe­lom bifurcates to form an afferent ring vessel which gives a vessel to each tentacle.

The tentacular vessels unite with the efferent ring vessel which in turn is connected with the ventro-lateral vessels or efferent vessels. Aborally, the dorso-median and ventro­lateral vessels communicate with the haemal plexus of the gut wall.

7. Excretory System of Phoronis:

The excretory system comprises of a pair of nephridia, which is also called metanephridia. Each nephridium is a ‘U’ shaped tubular structure and is lined through­out its length by ciliated epithelium. It opens to the exterior through nephridiopore on the side of the anus. The nephridia also act as gonoducts.

8. Nervous System of Phoronis:

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 a 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 (a cylindrical giant nerve fibre covered by a sheath) is present. In most of the species lateral nerve is present on the left side, ex­cept in a few species where it may also be present on the right side. The lateral nerve innervates the trunk musculature.

In Phoronis, sense organs are represented only by neurosensory cells.

9. Reproductive System of Phoronis:

Phoronis is unisexual or hermaphrodite. Specialised cells on the inner coelomic wall produce sperms and ova. Fertilization oc­curs either in the body cavity or in the lophophoral cavity where reproductive cells are brought out through the nephridiopores.

10. Development of Phoronis:

After the completion of fertilization, de­velopment occurs in the cavity enclosed by the tentacles. The development is indirect. Cleavage is holoblastic, equal and typically radial. The cleavage results into the forma­tion of coeloblastula. The vegetal 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 larval form is pro­duced. The larva is a free-swimming form and has an oval body. The pre-oral lobe becomes expanded and bent forward.

The alimentary canal is differentiated into oesophagus, stomach and intestine. As the larva develops, the cilia become restricted to certain areas of the body. The fully devel­oped larva is called Actinotroch larva.

It has an elongated form ranging from 1-5 mm. Overhanging the mouth of the larva there is a hood-like hollow pre-oral lobe and a circlet of ciliated tentacles (Fig. 20.1D, E). 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. Another post- oral ciliated ridge is present below the mouth. A girdle of slender tentacles originating 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.