Phoronids: Taxonomic Retrospect, Features and Affinities

In this article we will discuss about Phoronids:- 1. Taxonomic Retrospect of Phoronids 2. Characteristic Features of Phoronids 3. Fossil History 4. Geographical Distribution 5. Affinities.

Taxonomic Retrospect of Phoronids:

1. The phoronids constitute a very im­portant group amongst the lophophorate coelomates. The name of the group as Phoronida was first coined by Hatschek in 1888.

2. Another name, Phoronidea, proposed by Lang has not/been universally accepted.

3. Before the establishment of group, many workers studied the biology of many Phoronids.

4. J. Muller and his student, Wegener in the year 1847 first observed numerous free-swimming animals on the surface of the sea.

5. Both the workers wrongly took them as adult forms and named them as Actinotrocha branchiata. But their larval nature was pointed out subsequently by Gagenbaur (1854).

6. Wright (1856) first observed an aggre­gate of many adult worms inhabiting the tubes and called the animals as Phoronis.

7. Kowalevsky (1867) and Metschnikoff (1871), by following the developmen­tal sequences, have established the fact that the Actinotrocha larva metamor­phoses into adult Phoronis. This obser­vation led to the fact that the Phoronis is the adult worm and the Actinotrocha is its larval stage.

Characteristic Features of Phoronids:

1. Phoronids are exclusively marine.

2. Bilaterally symmetrical, sedentary coelomate worms of trimeric construc­tion.

3. Sessile, vermiform body enclosed in a leathery tube.

4. Body differentiated into three distinct parts:

(i) Prosome

(ii) Mesosome and

(iii) Metasome, each containing its coelomic cavity.

The prosome forms the epistome and mesosome bears the lophophore with the mouth and two rows of tentacles. The metasome or trunk is slender and cylindrical with a bulb-like posterior end called ampulla.

5. The lophophore is horse-shoe shaped with spirally coiled lateral cornu.

6. U-shaped alimentary canal and with anus outside lophophore.

7. Closed blood vascular system con­taining red blood corpuscles with hae­moglobin. Distinct heart is absent.

8. Excretory system contains a pair of metanephridia with ciliated nephrostomes, located on either side of intes­tine.

9. Nervous system consists of nerve fi­bres and nerve cells that lie within epithelial tissue.

10. Sexes separate or hermaphrodite but also can reproduce asexually.

11. Development indirect.

12. Cleavage holoblastic, equal and typi­cally radial.

13. Larva is called actinotroch larva.

There are two genera with 10 species.

Fossil History of Phoronids:

There is a poor fossil record for soft- bodied phoronids. They are known from Devonian.

Geographical Distribution of Phoronids:

Worldwide distribution, ranging from intertidal zone to about 400 m.

Examples:

Phoronis and phoronopsis.

Affinities of Phoronids:

The biological status of the Phoronida in the animal kingdom is a disputed issue. Because of the peculiar anatomical organisa­tion and structural resemblances with other groups, Phoronida has been a controversial subject since its discovery. Amongst all other groups, Brachiopoda and Ectoprocta resem­ble very closely with the Phoronida.

The Phoronida, Brachiopoda and Ectoprocta are collectively called the lophophorate coelomates. On the basis of their similarities, these three groups were regarded by earlier Zoologists, specially by Milne-Edwards (1843) and T. H. Huxley (1853) as a single phylum Molluscoidea. But due to their wide structural divergences and lack of mutual relationships, the scheme has been totally abandoned.

Caldwell (1882) also established the relationship between the three groups. Hatschek (1888) placed them in different classes under the phylum Tentaculata. Such attempt of combining them under one phy­lum seems to be inappropriate.

Hyman (1959) suggested an alternate name, Lophophorata for these groups because of the presence of lophophore. In spite of all attempts to in­clude them under one group, it is improper to draw any definite affinity between them, because there are many structural differ­ences. This aspect will be clear from the following discussion.

Relationship with Brachiopoda:

The Phoronida exhibits close resem­blances with Brachiopoda.

The similarities are:

1. Presence of similarly constructed horseshoe-shaped lophophore.

2. Presence of an epistome.

3. Presence of a U-shaped alimentary canal.

4. A coelomic septum is present separat­ing the mesosome and metasome, al­though poorly developed in Brachio­poda, except on Crania where the septum is complete.

5. Presence of sub-epidermal nervous plexus which forms a nerve centre in the mesocoel.

6. A pair of metanephridia is present. These are the coelomoducts of metacoel and also act as gonoducts.

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

8. In both the cases the dorsal surface between mouth and anus is extraordi­narily shortened.

Dissimilarities:

The Phoronida and Brachiopoda, though possess many similar features, have many structural differences which do not support the supposed affinities.

The main differences are:

1. The nerve centre is supraenteric in Phoronida but 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. But in Brachiopoda, larval tentacles are lacking.

3. The brachiopod shell cannot be corre­lated with the exoskeleton of Phoronida.

4. The chitinous setae in larval and adult Brachiopoda have no counterparts in Phoronida.

5. The circulatory system in Phoronida is more developed than in Brachiopoda. In Phoronids the blood vascular sys­tem is composed of closed blood ves­sels, whereas in Brachiopods it is of open type.

6. In Phoronida, the cleavage pattern is spiral, but in Brachiopoda cleavage is not spiral.

Remark:

Though there are many similarities in both cases, but in the anatomy and development there are some differences for which the two groups should not be included under a common group.

But according to Nicolson (1985), that the phoronids are often suggested as the ancestor of Brachiopods because the embryology of these two lophophorate phyla is somewhat similar and both have mono-ciliated tentacular cells within upstream ciliary collecting sys­tem.

Relationship with Ectoprocta:

The relationship between Phoronida and Ectoprocta has also been emphasised by Caldwell (1882).

The idea was based on the existence of the following similar features:

1. The nerve centre is situated in the mesocoel and it supraenteric in both.

2. The lophophore is horseshoe-shaped.

3. Epistome is present.

4. Alimentary canal is bent to form a U-shaped tube.

5. The disposition of coelom is similar and a septum separates the mesocoel from the metacoel.

Dissimilarities:

But a detailed study of the two groups shows much structural dissimilarity. They differ widely from both anatomical and embryological point of view. The following points stand as barriers to draw the relation­ship between them.

They 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.

Remarks:

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 by the following points:

(1) Muscular ver­miform body with cresentic lophophore.

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

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

(4) Trochophore type of larva has protonephridia.

Relationship with Annelida:

The relationship between Annelida and Phoronida is mainly based on the larval similarities. Because of their resemblances, the Actinotrocha is regarded by many au­thors to be a modified Trochophore larva.

The points of resemblances are:

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

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

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

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

(b) 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 the telotroch is similar in both.

(c) A thickening of the ectoderm of the preoral lobe in Actinotrocha represents (sometimes bearing eyes) the apical plate of Trochophore.

(d) A pair of solenocytic nephridia is present.

(e) 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 mesoderm is endomesodermal. The body of Phoronida is un-segmented, but in Annelida segmentation is the main criterion in their organisation. In Trochophore mesoderm is arranged in teloblastic bands.

Remarks:

The relationship between Annelida and Phoronida cannot be justified by criti­cal dxamination, but the larval resem­blances are quite striking. These larval similarities may be due to their adap­tive convergence.

Relationship with Hemichordata:

Many workers, specially A. Masterman (1897), tried to establish the relationship between Phoronida and Hemichordata. The relationship is primarily based on the simi­larities in the larval forms, between the Actinotrocha larva and the Tornaria larva of Hemichordata.

The similarities are:

1. The body division of Hemichordata (Proboscis, Collar and Trunk) corre­sponds to the body division of the 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 the tentaculate lophophore is similar to the tentaculated arms of Cephalodiscus, a member of Hemichordata.

5. Presence of superficial similarities in the disposition of larval coelom.

Dissimilarities:

A thorough analysis reveals many differ­ences.

The differences are:

1. The three divisions of the body of Phoronida are not justified by embryological data. Actually, Phoronida has two divisions in the body. The epistome of Phoronida is not a body region and does not con­tain coelom like that of proboscis of Hemichordata.

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

3. The coelom in Actinotrocha is divided into three compartments, but in Trochophore, the collar coelom and trunk coelom are paired.

4. The notochordal nature of the glandu­lar pockets in Phoronida is difficult to interprete.

Remarks:

From the above points it is quite appar­ent that most of the arguments for­warded by Master-man to establish the relationship between the Phoronida and Hemichordata are not corroborated by embryological facts.

Most of the points of resemblances are based on assump­tion save for the septum between the middle and posterior sectors of the body and the similarity between the tentaculate lophophore and the ten­tacular crown in Cephalodiscus. Because of the similarities it may be suggested that the two groups inherited these characters from a remote common an­cestor.