In this article we will discuss about Phylum Pogonophora:- 1. Taxonomic Retrospect of Phylum Pogonophora 2. Characteristic Features of Phylum Pogonophora 3. Classification 4. Affinities 5. Phylogenetic Relationship.
Taxonomic Retrospect of Phylum Pogonophora:
1. The first species of Pogonophore was collected in Indonesia in the year 1900.
2. Uschakov, a Russian Biologist in the year 1933, collected some specimens from the depth of 3500 metres in Okhotsk Sea.
3. Uschakov described the animals and placed them in the family Sabellidae under the phylum Annelida.
4. Johannson (1939) pointed out the difficulties to place them under Annelida and created a new group Pogonofora (correctly spelt now as Pogonophora).
5. Reisinger (1938) and Ulrich (1949) considered Pogonophora as a separate phylum and placed it between Lophophorates and Hemichordates.
6. Ivanov (1955) proposed the name Brachiata for the phylum, but the term Pogonophora has been retained.
Characteristic Features of Phylum Pogonophora:
1. Marine and benthic animals.
2. Sedentary, tubiculous animals.
3. Elongated, cylindrical, worm-like body is differentiated into three regions— protomesosome, metasome and opisthosoma.
4. The anterior end of protomesosome bears one to thousands of tentacles (the number various in different species); hence called Pogonophora or beard worms.
5. The middle part of the body behind the protomesosome which includes most of the length of the body called trunk or metasome that bears girdles of setae, seen in some species.
6. The posterior part of the metasome is called opisthosoma which is composed of nearly 95 segments and half of the segments bear setae.
7. A typical gut is lacking in the adult.
8. A trophosome which contains closely packed chemosynthetic symbiotic bacteria is present.
9. Coelom is without peritoneum.
10. Fertilization is external in Phylum Pogonophora.
11. Cleavage holoblastic and spiral.
12. A trochophore larva is seen in the developmental stages.
Classification of Phylum Pogonophora with Characters:
The pogonophores are classified under two orders:
Athecanephria and Thecanephria.
Order 1. Athecanephria:
The tentacles are free. The protosome and mesosome are distinct. Ventral adhesive papillae are lacking from the posterior part of the trunk. Nephri-diopores are laterally placed. Pericardial sacs are present, e.g., Siboglinum, Oligobrachia, Birsteinia.
Order 2. Thecanephria:
In most of that cases the tentacles are fused at their basal ends. The protosome and mesosome are mostly combined to form the protomesosome. Ventral adhesive papillae are present in the posterior part of the trunk. Nephridiopores are medially placed. Pericardial sacs are absent.
Examples:
Lamellisabella, Spirabrachia, Heptabrachia, Polybrachia, Calathealinum, Galathea.
Recent Zoologists divide the pogonophorans into two subclasses 1. Perviata and 2. Vestimentifera under the class pogonophora. Here pogonophora is treated as a class instead of phylum.
Affinities of Phylum Pogonophora:
The systematic status of Phylum Pogonophora is still disputed.
However, on the basis of some similarities such as:
(i) Retention of blastopore,
(ii) Enterocoelous development of coelom and
(iii) Division of coelom into protocoel, mesocoel and metacoel, it has been suggested by many that the pogonophores may be related to other enterocoelous coelomates (Deuterostomia)—the Echinodermata and Chordata. The relationship between Pogonophora and Hemichordates is quite striking.
Relationship between Pogonophora and Annelida:
(i) From the developmental studies of Pogonophora, it is known that the nerve cord is ventral so that the Pogonophorans would seem to be protostome.
(ii) The setae are present at the opisthosomal segments of the body which indicate similarity to those of annelids.
(iii) Presence of nephridia.
(iv) The segmentation of the mesoderm.
Relationship between Pogonophora and Hemichordata:
The Pogonophora and Hemichordata show many points of resemblances.
These are:
1. Undivided protocoel with a pair of coelomoducts and external apertures,
2. Similar pericardial sac,
3. A septum dividing mesocoel and metacoel,
4. Gonad situated in the trunk,
5. Intra-epidermal nervous system and
6. Absence of no peritoneal lining in the coelom.
But there are many points of dissimilarities between these two groups which make it difficult to draw the affinities between them.
They are:
1. Lack of adhesive papillae in Hemichordata,
2. Absence of gill-slits in Pogonophora,
3. Localisation of nervous mass in the protosome in Pogonophora and in mesosome in Hemichordata,
4. Position of heart is ventral in Pogonophora and dorsal in Hemichordata,
5. Different position of gonopores and
6. Lack of homology between the tentacular apparatus of the two groups.
Phylogenetic Relationship of Phylum Pogonophora:
Phylum Pogonophora exhibits close similarities with Hemichordata but because of the differences discussed above it will be difficult to include them under Hemichordata. However, it may be suggested that the similarities between pogonophores and hemichordates are due to remote phylogenetic connection.
From the above discussion it is convenient to place the pogonophores under a separate and independent phylum—Pogonophora. The similarities of the pogonophores with the Annelida indicate that the group is to be protostqme and closely related to the Polychaeta.