Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts
Normal Histology Digestive System
Food particles that are gathered by the gills are moved to the labial palps which act as a device to sort and direct food particles to the mouth. Two pairs of labial palps, each pair located on either side of the body, occur at the anterior end of the geoduck clam on either side of the gills (see label "lp" in anatomy sketch and histological overview). The labial palps are mainly composed of vesicular connective tissue with basophilic gland cells (probably mucus secreting) within the epithelium along the inner margins. The inner surfaces are ridged and ciliated and act as a food particle sorting device (Fig. 1a and 1b). Food enters the mouth and is conducted through the tubular, ciliated esophagus to the stomach (Fig. 1c). The epithelial cells of the esophagus are larger, and have longer cilia than the epithelial cells lining the stomach (Fig. 1d).
Figures 1a to 1d. Sections through the anterior end of the digestive tract of juvenile geoduck clams. Hematoxylin and eosin stain.
The digestive gland surrounds the stomach and is composed of numerous tubules which are formed at the distal ends of branching ciliated ducts. Nutrients from the stomach enter the tubules through these ciliated ducts (Fig. 2a) and are absorbed by the glandular cells lining the tubules and digested intracellularly. The crystalline style extends from the style sac into the stomach (Fig.2b). The style contains amylase which helps digest the starches present in the food. In addition, the style usually (in other bivalves) harbours many spirochete bacteria that are thought to secrete additional digestive enzymes. These symbiotic bacteria have not yet been detected in Panopea abrupta. Wastes from the digestive gland tubules are returned to the stomach and are eventually swept into the intestine (Fig. 2c).
Figures 2a to 2c. Sections through the digestive gland region of juvenile geoduck clams. Hematoxylin and eosin stain.
The excreta (wastes from digestion) proceed through the loops of intestine in the gonad (Fig. 3a to 3c). The wall of the intestine is composed of ciliated epithelium and mucus glands (Fig. 3d). The intestine continues dorsally and becomes the rectum.
Figures 3a to 3d. Sections through loops in the intestinal tract located between the digestive gland and gonad. Hematoxylin and eosin stain.
The rectum extends through the heart (Fig. 4a) and pericardial cavity (Fig. 4b) dorsal to the kidney and passes posterior to the posterior adductor muscle (Fig. 4c). The rectum opens through the anus at the posterior region of the suprabranchial space. The epithelial cells of the anus have elongate cilia (Fig. 4d).
Figures 4a to 4d. Sections through the hindgut of juvenile geoduck clams. Hematoxylin and eosin stain.
As observed in the connective tissue of the labial palps (Fig. 1b above), glandular tissue also occurs in the connective tissue adjacent to the epithelial layer of the rectum at the anus (Figs. 5a and 5b).
Figures 5a and 5b. Sections through the glandular tissue associated with both ends of the digestive tract of juvenile geoduck clams. Hematoxylin and eosin stain.
Barnes, Robert D., 1968. Invertebrate Zoology, 2nd ed. W. B. Saunders Company, Toronto, 743 pp.
Morse, M.P. and Zardus, J.D. 1997. Bivalva. Microscopic Anatomy of Invertebrates Vol. 6A Mollusca II. F.W. Harrison and A.J. Kohn. Wiley-Liss. pp. 7-118.
Bower, S.M. and Blackbourn, J. (2003): Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Normal Histology - Digestive System.
Date last revised: August 2020
Comments to Susan Bower
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