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After reading this article you will learn about the section of different invertebrates.
1. Scypha (= Sycon) sp. (Sponge):
A. Longitudinal section:
(Fig. 38.1)
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1. Presence of ostia, incurrent canals, prosopyles, radial canals, apopyles, ex-current canals, gastric oscula in the wall; spongocoel at the centre, and osculum at the tip.
2. Body wall made up of two layers of cells, ectoderm and endoderm.
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3. Gelatinous mesenchyme present in between the cell layers.
4. Incurrent and radial canals parallel; radial and ex-current canal in the same long axis; ex-current canal and spongocoel at right angle to each other.
5. Ostia, prosopyle, apopyle, gastric osculum and osculum communicates exterior to incurrent canal, incurrent canal to radial canal, radial canal to ex-current canal, ex-current canal to spongocoel and spongocoel to exterior respectively.
6. Incurrent canals, ex-current canals and spongocoel lined by flat cells.
7. Radial canals lined by collar cells.
8. Mesenchyme contains different types of spicules and cells.
9. Spicules are monaxon, triaxon and tetraxon.
10.Some spicules project beyond body wall.
B. Transverse section:
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(Fig. 38.2)
1. Outline circular.
2. Presence of incurrent canals, prosopyles, radial canals, apopyles, ex-current canals, gastric oscula and spongocoel.
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3. Body wall made up of two layers of cells, ectoderm and endoderm.
4. Gelatinous mesenchyme present in between the cell layers.
5. Incurrent and radial canals parallel- radial and ex-current canal in the same long axis; ex-current and spongocoel at right angle to each other.
6. Ostia, prosopyle, apopyle and gastric osculum communicates exterior to the incurrent canal, incurrent canal to radial canal, radial canal to ex-current canal and ex-current canal to spongocoel respectively.
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7. Incurrent canals, ex-current canals and spongocoel lined by flat cells.
8. Radial canals lined by collar cells.
9. Mesenchyme contains spicules and cells.
10. Spicules are monaxon, triaxon and tetraxon.
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11. Some spicules project beyond body wall.
2. Hydra sp.:
A. Longitudinal section:
(Fig. 38.3)
1. Long and narrow, with a flat basal disc at one end and a conical hypostome and tentacles at the other end.
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2. Coelenteron, a large cavity, enclosed by the body wall.
3. Mouth, a narrow opening at the tip of the hypostome.
4. Body wall consists of two cell layers, ectoderm and endoderm.
5. Mesoglea thin, non-cellular and present between ectoderm and endoderm.
6. Epitheliomuscular cells, interstitial cells, cnidocytes (cnidoblast cells), basal disc, sensory and nerve cells constitute ectoderm.
7. Nutritive muscular and gland cells constitute endoderm.
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In addition to the above:
(a) A lateral bud, structurally similar to hydra but without basal disc, or
(b) Testis, or
(c) Ovary, may be present.
B. Transverse section:
(Fig. 38.4)
1. Outline circular.
2. A large cavity, coelenteron, enclosed by body wall.
3. Body wall consists of two cell layers, ectoderm and endoderm.
4. Mesoglea, thin, non-cellular and present between ectoderm and endoderm.
5. Epitheliomuscular cells, interstitial cells, cnidocytes (cnidoblast cells), sensory and nerve cells constitute ectoderm.
6. Nutritive muscular and gland cells constitute endoderm.
3. Ascaris Sp. (Round Worm):
Transverse section (middle region):
(Fig. 38.5)
A. Female:
1. Circular in outline.
2. Body wall consists of cuticle, ectoderm and longitudinal muscular layer.
3. Cuticle thick and transparent.
4. Ectoderm syncitial.
5. Longitudinal muscles arranged in four bands, two dorsolateral and two ventrolateral.
6. A cell of the longitudinal muscle is differentiated into a contractile and a nuclear part.
7. Four inward projections of ectoderm, two lateral, one dorsal and one ventral.
8. Pseudocoel large.
9. Intestine somewhat dorsally placed and made up of columnar cells.
10. Sections of ovary, oviduct and uterus present.
B. Male (Points 1-8 same as in female):
1. Intestine somewhat ventral in position and made up of columnar cells.
2. Sections of testis, vas deferens and seminal vesicle present.
4. Neanthes (= Nereis) Sp.:
Transverse section (middle region):
(Fig. 38.6)
1. Dorsoventrally compressed and bears two lateral projections, the parapodia.
2. Body wall consists of cuticle, epidermis and muscle layers.
3. Cuticle thin with a network of fine lines.
4. Epidermis, a single layer of columnar cells, some of which are glandular.
5. Ventral epidermis thickened.
6. Circular muscle layer—outer narrow and continuous.
7. Longitudinal muscles arranged in four bundles, two dorsolateral and two ventrolateral.
8. Parietal epithelium, a single layer of cells lining the inner surface of the body wall.
9. Two pairs of oblique muscles and parapodial muscles present in each segment.
10. Dorsal and ventral blood vessels, ventral nerve cord and intestine present.
11. A splanchnic layer and a layer of columnar cells constitute intestine wall.
12. Each parapodium divided into two lobes, notopodium and neuropodium.
5. Pheretima sp. (Earthworm):
Transverse section:
A. Pharyngeal region:
(Fig. 38.7)
1. Circular in outline.
2. Body wall consists of cuticle, epidermis and muscular layers.
3. Cuticle thin and transparent.
4. Epidermis, a single layer of columnar cells, some of which are glandular.
5. Circular muscular layer outer, narrow and continuous.
6. Longitudinal muscle layer inner, thick and the muscles are arranged in bundles.
7. Presence of dorsal and ventral blood vessels and ventral nerve cord.
8. Pharyngeal wall constituted by an inner ciliated epithelium, muscles and gland cells.
9. Radial muscles run from pharynx to body wall.
10. Pharyngeal lumen compressed and divided into a dorsal salivary and a ventral conducting chamber.
B. Intestine: Typhlosolar region:
(Fig. 38.9)
1. Slightly laterally compressed.
2. Body wall consists of cuticle, epidermis and muscular layers.
3. Cuticle thin and transparent.
4. Epidermis, a single layer of columnar cells, some of which are glandular.
5. Circular muscular layer outer, narrow and continuous.
6. Longitudinal muscle layer inner, thick and the muscles are arranged in bundles.
7. Presence of dorsal and ventral blood vessels, ventral nerve cord and septal nephridia.
8. Intestinal wall constituted by an outer splanchnic layer, middle circular and longitudinal^ muscles and inner epithelium lining the gut.
9. The gut epithelium thrown into a dorsomedian fold, the typhlosole, supported by longitudinal muscles, and hangs downward in the lumen.
6. Hirudinaria Sp. (Leech):
Transverse section Crop region:
(Fig. 38.10)
1. Slightly dorsoventrally compressed.
2. Body wall consists of cuticle, epidermis, dermis and muscle layers.
3. Cuticle thin and transparent.
4. Epidermis, a single layer of hammer- shaped cells, the head directed outwards.
5. Dermis thick, made up of fibrous connective tissue and muscle fibres.
6. Circular-muscle layer outer, narrow and continuous.
7. Longitudinal muscles inner, discontinuous and arranged in striae.
8. Dorsoventral, oblique and vertical muscles present.
9. Coelomic cavity occupied by botryoidal tissue.
10. Crop bears two diverticula.
11. Crop wall consists of circular and longitudinal muscle fibres and epithelium thrown into short folds.
12. Dorsal, lateral and ventral haemocoelomic channels, ventral nerve cord and nephridia present.
7. Lamellidens Sp. (Fresh Water Mussel):
Gill lamina:
Transverse section:
1. Ladder-like in appearance.
2. Lamellae two, outer and inner.
3. Gill lamella composed of gill filaments connected by interfilamenter junctions.
4. Cells of gill filaments and interfilamenter junctions columnar and ciliated.
5. Ostia present in interfilamenter junctions.
6. The two lamellae connected by transverse inter-lamellar junctions.
7. Water tubes enclosed by gill lamellae and interlameller junctions.
8. Asterias Sp. (Star Fish):
Arm:
Transverse section:
(Fig. 38.11)
1. Semicircular; oral surface flat, aboral surface arched.
2. Surface bears spines, tubercles, warts, pedicellariae and branchiae.
3. Body wall consists of cuticle, epidermis, dermis, muscle layers and coelomic epithelium.
4. Cuticle of moderate thickness.
5. Epidermis consists of ciliated columnar, neurosecretory and gland cells.
6. Dermis; a fibrilar layer and contains ossicles and haemocoelomic spaces.
7. Muscle layers two; outer circular, inner longitudinal.
8. A-shaped ambulacral groove supported by ambulacral ossicles present on the oral surface.
9. Rows of tube feet present in the ambulacral groove.
10. Pyloric caeca, gonad and ampullae present in the large perivisceral cavity.
