Four different appendages in palaemon according to their function

There are 19 pairs of appendages in palaemon. Structurally they are of biramous type but show considerable variations depending on the functions they perform.

Basically, each biramus appendage consists of a common base protopodite, bearing two ramian, inner endopodite and outer exopodite.

Typically, the basal protopodite is bisegmented, proximal coxa for attachment with the body and a distal basis. Basis further bears two rami, both comprises of several different segments or podomeres, according to their function.

(1) Cephalic Appendages:

They are 5 in number and are of the following types:

1. Antennules:

Below the orbital notch, one on either side are the first appendages, the antennules. The protopodite consists of segments, proximal precoxa, middle coxa and distal basis.

Precoxa bears the opening of statocyst on its dorsal side. It also bears one basal spiny lobe called stylocerite, Basis is elongated and bears two long jointed feelers. Outer feeler is bifurcated. All feelers bear sensory setae and are tactile in function.

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2. Antennae:

Just behind the antennules a pair of antennae, the second appendage one on either side is situated.

The coxa of protopodite is swollen due to the presence of coxal gland (excretory) with its renal aperture. Basis bears a spine.

Endopodite is represented by a many jointed feeler, while exopodite is in the form of a broad and leaf like plate, the squama or scale.

It serves for balance during swimming. Thus the antennae are sensory, excretory and balancing in function.

3. Mandibles:

These are the third pair of appendages. Coxa is the main part differentiated into a proximal, triangular and hollow apophysis and a distal head.

Head forms a stout molar process bearing 5 or 6 dental plates and a plate like incisor process ending in 3 teeth.

Outer margin of head carries a mandibular palp made of 3 segments. The proximal segment represents the basis while two distal segments represent the endopodite. Mandibles constitute the biting jaw and are masticatory in function.

4. Maxillula:

These are small thin and leaf like fourth pair of appendages. Free borders of coxa and basis are covered with pointed spines and project inwards as jaws or gnathobases (Gr. gnathos : jaws).

Endopodite forms a curved process bifurcated at the apex. The exopodite is absent. It helps in the manipulation of food.

5. Maxillae:

These are last cephalic appendages and are thin and leaf like. The small coxa is partially divided while the large basis forms a bifurcated gnathobasis internally.

Endopodite is quite small while exopodite forms a large fan-shaped scaphognathite or baler. Its movement creat a water current passing over the gills. Maxillae help in respiration and in the manipulation of food.

(2) Thoracic appendages:

There are 8 pairs of thoracic appendages of which 3 pairs are maxillepedes and 5 pairs of paraeopods or walking legs.

1. First maxillipedes:

These are thin and leaf-like. Inner border of coxa and basis form endites or gnathobases. Outer side of coxa bears a bilobed respiratory primitive gill or epipodite.

Margins of exopodite and endopodite are fringed with setae. It is tactile and respiratory in function.

2. Second Maxillepedes:

Coxa is short bearing setae on the inner side and a small epipodite with a gill on the outer side. Basis is short bearing exopodite and endopodite.

Endopodite consists of five podomeres

(i) Ischium

(ii) Merus

(ii) Corpus

(iv) Propodus and

(v) Dactylus.

The last two segments are bent inwards, forming cutting plates. Exopodite is long slender and unjointed provided with setae. They help in feeding and respiration.

3. Third Maxillepedes:

In appearance these are leg-like and resembles 2^nd maxillepede. The coxa is short and bears an epipodite on its inner surface.

Basis is also short carrying exopodite and endopodite. Endopodite consists of three segments

(i) fused ischium and marus

(ii) Carpus

(iii) fused propodus and dactylus.

Exopodite is long, unjointed with setae. It helps in feeding and respiration.

4. Walking legs:

Out of five pairs of walking legs, first two pairs are chelate i.e., provided with chela or pincer and last 3 pairs are non-chelate i.e., without chela or pincer.

(3) Typical walking leg:

It is a non-chelate leg. Protopodite consists of coxa and basis, while endopodite consists of 5 podomeres, i.e., ischium, merus, carpus propodus and dactylus.

All the seven podomeres are arranged in a linear order and are movably jointed together.

1. First and Second pair of walking legs:

They resemble the typical leg except propodus, which is long forming a chela with dactylus.

They catch the prey to put it into the mouth. They help in walking and offence and defence of the animal.

2. Third, fourth and fifth walking leg:

These nonchelate legs resemble the typical walking leg. In female prawn, the inner surface of coxa of third walking legs bears genital aperture, while in male prawn, the genital aperture is found on the arthrodial membrane between leg and thorax of fifth walking leg.

There is a genital aperture on the inner surface of coxa of third leg in females.

There is a genital aperture on the arthrodial membrane between leg and thorax of fifth leg in males.

(4) Abdominal Appendages:

There are six pairs of abdominal appendages known as pleopods or swimmerates.

1. Typical abdominal appendages:

A typical pleopod consists of a ring-like coxa and a cylindrical basis. Endopodite and exopodite are leaf-like. Endopodite bears a short, curved, rod-like structure with a knob-like head the appendix interna.

All the appendix interna of opposite appendages form a series of bridges which serve to carry the eggs. The outer surface of basis and the margins of endopodite and exopodite are provided with numerous setae.

2. First pleopod:

Appendix interna is absent and endopodite is greatly reduced in size. Rest of the structure is like typical appendages.

3. Second pleopod of male:

Second pleopod in female is typical. But, in the male there is an additional rod-like and setae bearing process, the appendix interna and endopodite. The rest of structure is same with typical.

4. Uropod:

On either side of telson is a flat, broad, plate-like structure, the uropod in the last segment of the body.

Coxa and basis are fused forming a triangular sympod. Exopodite is large, oar-shaped, divided into two by a transverse suture. Endopodite is flat, oval smaller than exopodite, provided with setae.