In this article we will discuss about Lamellisabella:- 1. Habit and Habitat of Lamellisabella 2. External Structures of Lamellisabella 3. Body Wall 4. Coelom 5. Digestive System 6. Circulatory System 7. Respiratory System 8. Excretory System 9. Nervous System 10. Reproductive System 11. Development.
Contents:
- Habit and Habitat of Lamellisabella
- External Structures of Lamellisabella
- Body Wall of Lamellisabella
- Coelom of Lamellisabella
- Digestive System of Lamellisabella
- Circulatory System of Lamellisabella
- Respiratory System of Lamellisabella
- Excretory System of Lamellisabella
- Nervous System of Lamellisabella
- Reproductive System of Lamellisabella
- Development of Lamellisabella
1. Habit and Habitat of Lamellisabella:
Like all other Pogonophores, Lamellisabella is exclusively marine and benthonic creature. They are sedentary, tubicolous and free-living animals. The exact mode of nutrition is not known and the digestive tract is wanting.
2. External Structures of Lamellisabella:
Lamellisabella has an elongated slender worm-like cylindrical body ranging from 50- 350 mm in length and 0.5 to 2.5 mm in diameter. This animal has a crown of tentacles and it bears a superficial resemblance with an Annelid, Sabella.
Such resemblance has given rise to the name for the genus Lamellisabella. Each worm lives in a chitinous tube formed by its own secretion. The tube is composed of linearly arranged funnel-like pieces (Fig. 17.45A). The tubes are closely fitted separate units and have erect position.
The body is differentiated into three regions—protomesosome, metasome and opisthosoma. The protomesosome is formed by the fusion of protosome and mesosome. The mesosome and metasome are internally separated by a septum. The anterior end of the proto-mesosome bears circlet of tentacles varying from 18 to 31 in number (Fig. 17.45B).
In other groups, the number of tentacles varies upto thousands. The tentacles are fused at the basal ends so as to form a sort of cylinder. Formation of this cylinder is a characteristic feature of this genus. The tentacles are hollow and are provided with pinnules. The pinnules of the tentacles form a food-catching net in the cylinder.
The longest part of the body is called trunk or metasome which is un-segmented and contains uninterrupted coelomic cavities. The metasome is slender and is provided with rows of adhesive papillae for anchorage in the tube.
The papillae are provided with minute hard horseshoe-shaped structures. Two oblique adjacent ridges, called belts, divide the trunk into preannular and postannular regions. The most posterior end of the body is called opisthosoma (Fig. 17.46) which is segmented and half of the segments bear setae. This part is used for digging in the sediment.
In the head of vestimentiferan pogonophores (Fig. 17.46), one of two anteriorly elongated, medially fused flat-topped structures that supports the gills, called the obturaculum.
3. Body Wall of Lamellisabella:
The body wall of Lamellisabella comprises of an epidermis. The epidermis is lined by cuticle on the outer side while a muscular layer is present on the inner side. The epidermis proper is composed of columnar and gland cells. The muscular layer is differentiated into a thin circular muscle layer and a thick longitudinal muscle layer.
4. Coelom of Lamellisabella:
The coelom is not lined by peritoneum. The coelom forms paired compartments in protomesosome and metasome.
5. Digestive System of Lamellisabella:
The absence of alimentary system has raised doubt about the mechanism of nutrition. The cylindrical space formed by the fusion of basal ends of the tentacles together with the pinnules form a food-catching net which plays the main role in nutrition. It is supposed that the ciliary tracts in the pinnules produce water current which carries microorganisms. These micro-organisms are entangled in the food-catching net.
The digestion and absorption of food also occur within the net. Thus it exhibits a peculiar instance where the food is digested outside the alimentary canal. The coelomic cavity of the trunk contains gonads, and central mass of tissue is called trophosome which contains closely packed symbiotic chemosynthetic bacteria that also help in nutrition.
6. Circulatory System of Lamellisabella:
The circulatory system of Lamellisabella is comparatively well-formed and is of closed type. It consists of one mid-dorsal, one mid-ventral and two pairs of lateral vessels in the trunk region. The mid-ventral vessel in protosome becomes enlarged into a muscular heart.
The pericardial sac is absent in this genus. The blood, which does not contain any cell, circulates posteriorly through dorsal vessel and anteriorly through ventral vessel. Each tentacle is provided with one afferent and one efferent vessel. These vessels form a loop in the pinnules.
7. Respiratory System of Lamellisabella:
No definite respiratory organ is present in Lamellisabella. The tentacles serve as the respiratory organs. The body of the worm is protruded anteriorly from the tube for the purpose of gaseous exchange.
8. Excretory System of Lamellisabella:
The excretory system of Lamellisabella includes two nephridia (modified coelomoducts). The nephridia are situated in the cavity of the protomesosome. Each nephridium is a sacciform structure which opens to the exterior through the nephridiopore (Fig. 17.45C). The nephrostome is absent. The excretory materials are accumulated by the nephrocytes in the coelomic fluid.
9. Nervous System of Lamellisabella:
The nervous system of Lamellisabella is very primitive in nature and is intra-epidermal in position. It is composed of a ganglionic enlargement in the cephalic lobe of protomesosome. This ganglionic enlargement gives nerve supply to the tentacles and a single mid-dorsal nerve cord emerges posteriorly.
10. Reproductive System of Lamellisabella:
The sexes of Lamellisabella are separate. The male and female can be distinguished by the position of the gonopores. The male gonopores are situated behind the protomesosome-metasome septum and the female gonopores are located at the middle region of the metasome. The gonads are elongated paired bodies situated in the metasome.
11. Development of Lamellisabella:
The eggs are large and yolky. Fertilization is external. The cleavage is holoblastic and results in the formation of a large number of blastomeres.
The embryo is bilaterally symmetrical and fusiform in shape. A rudiment of archenteron is formed in the embryonic stage, but it degenerates subsequently. The coelom is enterocoelous in origin. The embryo is transformed into an elongated larva which, after a brief free existence, transforms into adult.