External features of palaemon or prawn of phylum arthropoda

External features of palaemon or prawn of phylum arthropoda

Phylum Arthropoda is the largest phylum of the animal kingdom and crustacea is a large and important class of the phylum Arthropoda. It includes prawns, crabs, lobesters, shrimps, barnacles, water fleas and related forms.

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Habits and habitat :

Palaemon or prawn, found in fresh water ponds, rivers and lakes. It is a nocturnal animal. It comes out at the water surface in the night in search of food.

It is an omnivorous. It is an active swimmer and swims with the help of pleopods. It breeds from May to July.

Shape and Size:

Palaemon has elongated, spindle shaped, bilaterally symmetrical body. Different species have different sizes. P. malcomsonii (Macrobrachium malcomsonii) the giant prawn found in Tamilnadu, Bengal and Central India is about 30 cm in size.

Colouration:

Young stages are transiuscent and white but the adults are usually dull pale-blue or greenish with orange-red patches.

Segmentation:

Body of prawn is divided into two main regions – anterior cephalothorax and a posterior abdomen.

(a) Cephalothorax:

Cephalothorax is formed by the union of head and thorax region and consists totally of 13 segments.

Apparently it seems unjointed large and rigid. All segments of cephlalothorax bear jointed appendages.

(b) Abdomen:

It is jointed region with 6 distinct movable segments and a terminal conical piece, the telson.

These segments are dorsally rounded, laterally compressed and normally bent under the cephalothorax, so that the animai looks like (‘) comma shaped. Each abdominal segment bears a pair of jointed appendages called pleopods or swimmerets.

External Apertures

Mouth:

At the anterior end of cephalothorax is a mid-ventral slit-like aperture called mouth.

Anus:

At the base of the telson is a mid ventral longitudinal slit-like aperture, the anus.

Renal aperture:

These are paired apertures found on a papilla at the inner surface of coxa of antennae.

Male genital pores:

These are also paired apertures found at the inner surface of the coxae of 5^th pair of walking legs in males.

Female genital pores:

These are paired apertures at the inner surface of the coxae of 3^rd pair of walking legs in females.

Openings of statocysts:

At the dorsal surface of the precoxae of antennule paired statocysts open outside.