EXTERNAL CHARACTERS OF A PALAEMON (FRESH WATER PRAWNS)
Palaemon belongs to phylum Arthropoda, sub-phylum Mandibulata, clas Crustacea, sub-class Malacostraca, order Decepoda.
Palaemon is the fresh-water prawn. Palaemon inhabits fresh water streams, rivers, lakes and ponds in India. The dry prawns and fresh prawns are used in various recipies.It prefers clean fresh waters which move, slowly. It remains at the bottom by day, and comes to the surface at night and feeds on algae, moss, small insects and debris. It respires by gills.
Palaemon shows an elongated body which is spindle like and shows bilateral symmetry. It is 25 to 35 cm. long. The entire body is covered by a strong, cuticular exoskeleton. It gives protection against injuries and provides space for muscle attachment. It is formed of the scelerities, which are hardened by-deposition of calcium salts. They articulate with one another by thin arthrodial membranes.
The body is divided into two parts.
- Cephalothorax, and
1. Cephalothorax: The cephalothorax is large and unsegmented.
i) Segments: The cephalothorax shows 13 segments. First five appendage bearing segments constitute the head. The remaining eight appendage bearing segments form the thorax.
Exoskeleton: The cephalothorax is covered by dorsal sheild and the sternal plates. The dorsal shield further consists of the anterior dorsal plate and the posterior carapace. The dorsal plate is triangular. It represents the fused terga of the cephalic segments. It extends forward as rostrum. Below the base of the rostrum, the dorsal plate has on each side a notch (Orbital notch) in which the short stout eye-stalk is present. The eye-stalks carry eyes. The carapace covers the thorax dorsally and laterally. On either side of the body, the carapace hangs freely as a convex gill cover. The space between the thorax and the gill cover is called the chamber. Gills are present in the gill chamber.
The sternal plates form the floor of the cephalothoracic box.
ii) Appendages: The cephalothorax bears 13pairsof appendages. Of these, the anterior 5 pairs belong to the cephalic region, and are called the cephalic appendages. The remaining 8 pairs belong to the thorax and are called the thoracic appendages.
2. Abdomen: The abdomen is six segmented. It is almost circular in cross - section.
i) Segments: The abdomen consists of six segments and a flat conical telson.
ii) Exoskeleton: Each abdominal segment is covered dorsally by a convex sclerite, the tergum, and ventrally by a flat sclerite, the sternum. The tergum extends downwards on either side as pleuron. The ploura of 2nd segment are large and cover the pleura of the preceding segment. The pleura ofthe 6th segment are very small. The adjacent terga and stema are connected by arthrodial membrane called the intertergal and intersternal membranes respectively. The adjacent terga articulate by a hinge joint on either side.
iii) Appendages: The abdomen bears six pairs of appendages. These are known as the pleopods.
- The slit-like mouth opens midventrally at the anterior end of the cephalothorax.
- The anus is a longitudinal aperture present at the base of the telson, ventrally.
- The paired renal apertures open on the inner surface of the coxa of antennae.
- The paired female genital apertures open on the inner surface of coxa of the third pair of walking legs in the female.
- The paired male genital apertures are present on the inner surface of coxa of the fifth pair of walking legs in the male.
- The openings of the statocysts are on the basal segment of each antennule.