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In this article we will discuss about Nemertine:- 1. General Characters of Nemertine 2. Structure of Nemertine 3. Body Wall 4. Proboscis 5. Digestive System 6. Circulatory System 7. Excretory System 8. Respiratory System 9. Nervous System 10. Sense Organs 11. Reproductive System 12. Development 13. Affinities.
Contents:
- General Characters of Nemertine
- Structure of Nemertine
- Body Wall of Nemertine
- Proboscis of Nemertine
- Digestive System of Nemertine
- Circulatory System of Nemertine
- Excretory System of Nemertine
- Respiratory System of Nemertine
- Nervous System of Nemertine
- Sense Organs of Nemertine
- Reproductive System of Nemertine
- Development of Nemertine
- Affinities of Nemertine
1. General Characters of Nemertine:
The nemertines are non-parasitic un-segmented worms, most of which are marine, only a few forms living on land or in freshwater.
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Body slender, worm-like, soft, highly contractile, un-segmented, bilaterally symmetrical, triploblastic and acoelomate. The ectoderm is ciliated. Eversible proboscis lying in a sheath on the dorsal side of the gut. The gut is straight, complete with lateral diverticula and terminates in posterior anus. True coelom and respiratory organs are absent, body spaces are filled with gelatinous mesenchyme.
Circulatory system is closed. Paired, excretory tubules provided with flame cells. Nervous system comprises a well differentiated brain from which arises a pair of lateral longitudinal nerves. A pair of ciliated pits are also connected with the brain. Sexes are separate, gonads are simple sac-like structures. Development direct or through a free-swimming pilidium larva. Asexual reproduction by fragmentation.
2. Structure of Nemertine:
The body is nearly always narrow and elongated, cylindrical or depressed, un-segmented and devoid of appendages. In length they vary from a few millimeters to as much as twenty-seven metres. In some cases, there is a short narrower posterior region or tail. The distinct head is absent.
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The entire surface is covered with vibratile cilia and frequently the integument is vividly coloured.
Gland cells of the epidermis secrete a mucous matter, which may serve as a sheath or tube for the animal. The mouth is situated at or near the anterior extremity on the ventral aspect. Near the mouth in front there is an opening through which can be protruded a very long muscular organ, the proboscis. The proboscis is hollow.
3. Body Wall of Nemertine:
Body wall consists of epidermis, dermis, circular muscle layer and longitudinal muscle layer.
The outermost layer of the body wall is an epidermis of columnar cells, many of which are ciliated, while others are unicellular glands, some of which are arranged in groups. The unicellular glands secrete mucus with which the surface is usually covered and may form gelatinous tube. Beneath the epidermis is a basement membrane, very thin in most cases.
Then follows the dermis forming a connective tissue layer. Beneath dermis is a thick muscular layer. In some nemertines there are only two layers of muscle fibres, an outer circular and an inner longitudinal, in the rest a third (longitudinal) layer is superadded. The gland cells are scattered irregularly between the narrower portions of the ciliated columnar epidermal cells.
Besides the unicellular glands many nemertines have cluster of gland cells opening at the surface by a common duct, these are known as the packet glands. The packet glands may be included within epidermis as in paleonemertines or they may sink into the sub-epidermal tissue as in the heteronemertines.
4. Proboscis of Nemertine:
The proboscis is the most characteristic feature of nemertines. It is an elongated, coiled, hollow and muscular organ opening in front and closed behind. It lies within the proboscis sheath and retractile in nature. When retracted it lies within the body in a fluid-filled cavity called rhynchocoel which is completely shut off from the exterior.
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The muscular walls of the rhynchocoel form the proboscis sheath. The epithelial cells of the proboscis, in most cases, secrete rods identical with the rhabdites of Turbellaria. The blind end of proboscis is attached to the posterior end of the sheath by a retractor muscle which checks the eversion of the proboscis beyond a certain point, and by means of which also it is retracted.
The part of the proboscis in front of the brain is called rhynchodaeum which opens to the exterior through the proboscis pore. The proboscis may be armed or unarmed, if armed with styles at the tip. The lining epithelium of the proboscis closely resembles the surface epidermis from which it is derived.
The muscle layer of proboscis and those of the proboscis sheath repeat the body wall musculature. The proboscis is shot out with explosive force through muscular contraction exerting pressure in the fluid of the rhynchocoel. As this is done the proboscis turns inside out (everts) and protrudes. Withdrawal of the proboscis is brought about by retractor muscles.
5. Digestive System of Nemertine:
The mouth is situated ventrally near the anterior tip. The digestive tract is a ciliated tube which extends throughout the length of the body from the mouth to the anus. The first part of the digestive tract is usually a simple tube-oesophagus (stomodaeum) but may be more complicated and divided into various regions, sometimes with paired diverticula. Posteriorly it opens into the intestine.
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The intestine may be a simple un-constricted tube or may be only slightly constricted at intervals by the paired gonads. In most cases the constrictions corresponding to the gonads are very deep so that the intestine comes to be provided with two rows of lateral diverticula or caeca which may be branched. The caeca are separated from one another by incomplete transverse septa of dorsoventral muscular fibres.
The arrangement of the caeca and septa with the alternately arranged gonads bringing about an appearance of imperfect metamerism as observed in some of the Platyhelminthes. The intestine opens to the exterior by an anus at the posterior end of the body. The nemertines feed usually at night on living or dead animals mainly annelids, molluscs, crustaceans and fishes, etc.
6. Circulatory System of Nemertine:
The circulatory system of nemertines is closed type. There are three principal longitudinal trunks a median dorsal and two lateral. The longitudinal trunks lie in the parenchyma, one on each side of the intestine and one just above it. The lateral vessels communicate with each other both anteriorly and posteriorly by spaces lined by only delicate membranes respectively known as cephalic lacuna and anal lacuna.
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The longitudinal vessels give off other lateral branches that open into a system of lacunae in the tissues.
The blood is, in most cases, colourless and contains rounded or elliptical, usually colourless corpuscles. In some cases the blood is red because the corpuscles contain haemoglobin. Circulation in nemertines is primitive in several respects. There is no pumping organ or heart. The circulation is brought about by general movement of the body.
7. Excretory System of Nemertine:
The excretory system consists of a pair of longitudinal vessels which give off branches. Each longitudinal vessel opens to the exterior by a nephridiopore on each side. The fine terminal branches of the system are provided with ciliary flames each situated in the midst of a group of cells, not in the interior of a single flame-cell as in most cases in the flatworms.
8. Respiratory System of Nemertine:
There are no special organs of respiration in any of the group. But there is evidence that this function is carried out, in part at least, by taking in and giving out of water through the mouth by the oesophagus. Respiration also takes place by the diffusion of oxygen through the general body surface.
9. Nervous System of Nemertine:
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The nervous system (Fig. 53.4) consists of brain. The brain is composed of two pairs of ganglia, dorsal and ventral.
The ganglia of each pair being connected together by commissures, the dorsal situated above and the ventral below. From the brain pass backwards a pair of thick longitudinal nerve cords which run throughout the length of the body. Usually these are lateral in position, sometimes approximated dorsally and sometimes ventrally.
The lateral nerve cords generally meet posteriorly in a commissure usually situated above, but in one genus below, the anus. A third median dorsal nerve of smaller size than the lateral nerve cords extends backwards from the dorsal commissure of the brain. The position of brain and lateral nerve cords, or the system of commissures and nerve branches varies in different groups.
10. Sense Organs of Nemertine:
The sense organs of nemertine consist of sensory nerve cells, sensory pits and eyes. The special organs of the sense are mostly restricted to the anterior part of the body but the sensory nerve cells are found scattered in the epidermis of anterior as well as posterior ends.
The sensory nerve cells are slender-like in appearance, each bearing a hair-like process on the outer side. They are tactile in function. Sensory pits are found all over the body. Eyes are present in majority of nemertines and in the more highly organised species occur in considerable numbers.
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Sometimes they are of extremely simple structure, in other cases they are more highly developed, having a spherical refractive body with a cellular vitreous body and a retina consisting of layer of rods enclosed in a sheath of dark pigment. Each rod has a separate nerve branch connected with it. Statocysts containing statoliths have been found in only a few of the nemertines.
11. Reproductive System of Nemertine:
Most nemertines are dioecious but some may be hermaphroditic. The gonads are simple, tubular, sac-like structures, situated in the intervals between the intestinal caeca. The ovary or testis is a sac lined by cells which give rise to ova or spermatozoa; when these are mature each sac opens by means of a narrow duct leading to the dorsal, rarely to the ventral surface, or which it opens by a pore.
12. Development of Nemertine:
In nemertines the development is of two types:
(i) Direct
(ii) Indirect.
The characteristic larval form is the pilidium. This is a helmet-shaped body with side lobes like ear-lappets, and a bunch of cilia representing a spike.
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In the metamorphosis a number of ectodermal invaginations growing inwards around the intestine, fuse together and form the integument and body wall of the future worm, which subsequently frees itself from its investment and develops into the adult form. In the others there is a ciliated creeping larva called the larva of Desor in the interior of which the larval form is developed much as in the case of the pilidium.
13. Affinities of Nemertine:
The position of nemertines is very difficult to settle. They show affinities with lower chordates, vertebrates and the Platyhelminthes.
Affinities with Lower Chordates:
Nemertines possess several characters in common with a typical lower chordate, such as Balano gloss us.
These are:
(1) Elongated vermiform body.
(2) No external metamerism.
(3) Skin smooth, containing unicellular glands.
(4) Ectodermal nerve plexus.
(5) Terminal anus.
(6) Simple metamerically situated gonads.
(7) Retractile proboscis of nemertines is equivalent to the non-retractile proboscis of Balanoglossus.
Affinities with Vertebrates:
Nemertines are considered to be the ancestral forms of vertebrates on account of the following similarities:
(1) Dorsal nerve resembles the spinal cord of vertebrates.
(2) Lateral nerves resemble the nerves of lateral lines of fishes.
(3) Cerebral ganglion resembles the brain of vertebrates.
(4) Proboscis sheath suggests the notochord of vertebrates.
Affinities with Platyhelminthes:
Nemertines are often united with the Platyhelminthes. They are supposed to be related to the Turbellaria and were previously included in that class.
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The following similarities are found in both the groups:
(1) The shape of the body is flat, ribbon-like or tape-like without external segmentation.
(2) Body is completely covered with a ciliated epithelium containing gland cells.
(3) Just below the integument are thick, highly contractile muscles.
(4) The space between the body wall and the gut is filled with mesenchyme. The coelom is absent.
(5) Excretory system comprises flame cells.
(6) Nervous system is similar to that of flatworms.
However, nemertines differ from the flatworms in several features. Of these the most noticeable are:
(1) Defined body wall.
(2) Complete digestive tract with a second opening, the anus.
(3) Vascular system of higher organisation.
(4) Higher organisation of organs and tissues in general.
(5) Massive brain forming a ring around the digestive tract.
(6) Presence of proboscis independent of the gut.
On the basis of above similarities and dissimilarities it is advisable to place them in a separate phylum.
