Appendages of prawn. Penaeus sp.

Penaeus bears nineteen pairs of appendages.They include five cephalic, eight thoracic and six abdominal appendages .The appendages of prawn are many-jointed.The segments of an appendage are known as podomeres. The appendages are typically biramous (= two branched).

Each appendage has a two-jointed basal region called protopodite to which are attached two distal processes, the outer exopodite and the inner endopodite. This basic plan of the appendages is modified in different parts of the body to suit the varying functions performed by them.

Cephalic Appendages:

Head region bears five pairs of appendages. These include a pair of antennules, a pair of antennae, a pair of mandibles, first pair maxillae and second pair maxillae.

1.Antennules:They are the anterior most appendages situated below the level of the eyestalks. They are also known as first antennae. Each antennule has a proterpodite, an exopodite and an endopodite .Protopodite has three podomeres, the proximal precoxa, middle coxa and the distal basis. The precoxa has a hollow depression on one side. The eye and the balancing sense organ/ statocyst are located in it. The opening of the statocyst lies at the base of this podomere. The basis carries short many jointed flagella-like exopodite and endopodite. They function as tactile sense organs.

2.Antennae:They are also known as second antennae. They lie immediately behind the antennules. Each antennule consists of protopodite, exopodite and endopodite. Protopodite has two podomeres, the proximal coxa and distal basis. The opening of the excretory duct lies in the coxa. Exopodite and endopodite are situated on the basis. Endopodite has three basal podomeres and a long narrow many-jointed filament. It functions as tactile sense organ. The exopodite is a flat broad plate-like structure and it is also known as squame. It functions as a balancing organ.

3.Mandibles:They lie on either side of the mouth. Each mandible has a protopodite and an endopodite. Exopodite is absent. Protopodite is a single stout calcified structure with toothed inner edge.

The teeth are used for grinding food to size. The endopodite, which is attached to the protopodite is segmented. The endopodite is also known as sensory palp.

4.First pair maxillae (Maxillulae):They are the smallest appendages of prawn. Each maxilla has a protopodite and an exopodite.Exopodite is absent. Each protopodite has two flattened leaf-like lobes, the coxa and basis. They protect inwards as jaws and are also called gnathobases.Endopodite is uniointed and leaf-like with narrow distal end. Both gnathobases and endopodite are provided with sharp hair like processes.

5.Second Pair maxillae:They lie just behind the first Pair of maxillae. Eacir maxilla is provided with a protopodite, an exoPodite and an endopodite.Protopodite is flat and four lobed.Their edges are flattened and provided with hair-like recesses.They aid in mastication.The endopodite is small, unsegmented and leaf-iike. It is situated between the protopodite and the exopodite.The exopodite is large, expanded, flat and boat-shaped and is also known as scapho gnathite. Its movement producesWater  currrents in the gill chamber.

Thoracic Appendages:Thorax bears eight pairs of appendages. They. include maxillipeds and peraeopodes or walking legs.They have a protopodite of two podomeres, an endopodite of five podomeres and an unsegmented exopndodite. The podomeres of the protopodite are proximal coxa and distal basis. The podomeres of the endopodite are named from the base ischium, merus,corpus, propodus and dactylus.

Maxillipeds:The first three pairs of thoracic appendages are known as maxillipeds,or foot jaws. They are directed forwards and run parallel to the middle line of the body.

1.First maxillipeds:The first maxilliped has a foliaceous appearance. It has a protopodite, exopodite and endopodite. Protopodite is flat and incompletely divided into two small proximal lobes and a large distal lobe.They are provided with setose processes on their edges. The exopodite is flattened and leaf-like with a broad base. Attached to the protopodite and exopodite is a proximal, triangular, flat structure called epipodite. It is respiratory in function.

2.Second maxillipeds:Protopodite has two podomeres, the proximal coxa and distal basis . Exopodite and endopodite are attached to the basis. Endopodite is made up of five segments. Endopodite curves distally and gives the shape ot an interrogation mark. Exopodite is slightly flattened with striations and feathery edges. A Y-shaped epipodite is attached  to the coxa of the protopodite.

3.Third maxillipeds:This is similar to the second maxilliped, but the endopodite is straight with the five segments . Exopodite is flat with striations and feathery edges. Y-shaped epipodite is attached to the coxa of the protopodite.

Peraeopods (walking legs):There are five pairs of peraeopods. The first three pairs of peraeopods are known as chelate legs or chelipeds and the last two pairs of peraeopods are known as non-chelate legs. In all peraeopods, the exopodite is small and fringed with many hair-like structures.

4-6Chelate legs (Chelipeds):All chelate legs are identical in structure. But they differ slightly in size, the third one being the longest. The five jointed endopodite shows chelate articulation of the terminal two

podomeres, the propodus and the dactylus . By the hinged articulation of the dactylus to the side of the propodus, a pincer like apparatus is formed. This helps in grasping the food and passing it on to the mouth. The chelate legs are also used for walking

7-8.Non-chelate legs:In non-chelate legs, dactylus and propodus do not form chelate articulation . Epipodites are absent. Non-chelate legs are used for walking.

Abdominal appendages:There are six pairs of abdominal appendages.They are also known as pleopods or swimmerets. Each abdominal appendage consists of a protopodite and unjointed exopodite and endopodite. Protopodite is two-segmented and the exopodite is flattened and fringed with setose processes.

1.First pleopods:In female, exopodite is flattened, thick and fringed with setose processes. Endopodite is absent or may be present as a very small bud-like process. In male, endopodite is short and provided with hooks.The hooks of the  endopodites of the two sides interlock and form a rod-like structure called the petasma which is used for transferring sperms into the thelycum of female.

2-5.Second, to fifth pleopods:The second, third, fourth and fifth pairs of pleopods have a typical biramous structure, with a two-jointed protopodite, and unjointed exopodite and endopodite .

6.Sixth pleopods (Uropods):The sixth abdominal appendages are known as uropods. Each uropod has a protopodite, an exopodite and an endopodite. In protopodite the podomeres, coxa and basis fuse to form one segment. The two uropods and the telson form the tail-fin or tail-fan which acts as a balancing organ. Its sudden flexion causes backward leap.