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In this article we will discuss about Phylum Nemertea: 1. Features of Phylum Nemertea 2. Scheme of Classification of Phylum Nemertea 3. Phylogenetic Relationship.
Features of Phylum Nemertea:
1. Body of Phylum Nemertea is mostly slender; elongated and often flattened, hence called ‘ribbon worms’ or ‘proboscis worms’ (Fig. 14.31).
2. Most species of Phylum Nemertea are less than 20 cm in length and a few may be several metres or more.
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3. Phylum Nemertea has un-segmented body.
4. Anterior end pointed or spatula- shaped.
5. Epidermis ciliated and devoid of a cuticle.
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6. Circular, longitudinal or dorsoventral muscles beneath the epidermis.
7. Locomition by cilia or muscular peristalsis or by both.
8. There is no general body cavity (acoelomate) and the internal organs are surrounded by mesenchyme.
9. There is an unique eversible muscular, hollow, retractile proboscis apparatus used in capturing food (Fig 14.32A), hence called proboscis worms.
10. The proboscis may be simple tube or armed with a calcareous barb, called stylet (Fig. 14.32B).
11. Generally proboscis not connected with the digestive tract and proboscis apparatus floats in a separate fluid filled cavity, called rhynchocoel, homologous with the coelomic cavity.
12. Digestive tract with an anterior mouth and a posterior anus with a long tubular midgut and numerous paired lateral diverticula.
13. Digestion in Phylum Nemertea is extracellular and intracellular.
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14. True blood vascular system without heart present. Blood circulates through contractile vessels.
15. Haemoglobin in the blood of few nemerteans and in the nervous system of many species.
16. Excretion by single pair to several thousand pairs of protonephridia with flame cells.
17. Nervous system consists of 2 pairs of cerebral lobes and two primary and lateral longitudinal cords.
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18. Sense organs are ciliated pits and pigmented ocelli.
19. Sexes of Phylum Nemertea are separate mostly; a few are hermaphroditic.
20. Fertilization is external in Phylum Nemertea.
21. Cleavage spiral.
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22. Development typically direct, but in many heteronemerteans Pilidium larva occurs.
The phylum Nemertea is divided into two classes:
(i) Anopla and
(ii) Enopla.
Scheme of Classification of Phylum Nemertea:
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(According to Barnes 1980; Ruppert and Barnes 1994).
Phylum Nemertea (= Rhynchocoela)
Class Anopla
Order Palaeonemertea, Heteronemertea
Class Enopla
Order Hoplonemertea, Bdellonemertea
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Classification with Characters:
Class 1. Anopla:
1. Mouth located at the back of the brain.
2. Stylet absent in the proboscis.
It includes two orders:
Order 1. Palaeonemertea:
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1. Body wall musculature two to more layers.
2. No larval stage in the life cycle.
Examples. Carinoma, Tubulanus.
Order 2. Heteronemertea:
1. Three layered body wall musculature.
2. Pilidium larva in the life cycle.
Examples. Cerebralutus, Micrura, Lineus.
Class 2. Enopla:
1. Mouth situated at the front of brain.
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2. Generally stylet present in the proboscis.
It includes two orders:
Order 1. Hoplonemertea:
1. Stylet in the proboscis.
2. Mostly of freshwater and terrestrial forms.
Examples. Prostoma, Ototyphlonemertes, Gononemertes.
Order 2. Bdellonemertea:
1. Stylet absent secondarily.
Single genus Malacobdella with most species live in the mantle cavities of some marine bivalves.
Phylogenetic Relationship of Phylum Nemertea:
The phylogenetic relationship of Nemertea with the turbellarian flatworms indicate in several respects such as:
(i) Absence of a cuticle,
(ii) Ciliated epidermis,
(iii) Mesodermal parenchyma and
(iv) Presence of protonephridia and it is assumed that the Nemertea has evolved from acoelomate ancestor shared with flatworms.
Another view is that the molecular studies indicate the link between nemerteans with some coelomate groups, such as Sipuncula, Annelida and molluscs.
