Describe the respiratory system of palaemon.
Ans.
Prawn respires in the aquatic medium and it carries three sets of
organs for the purpose lining of the branchiostegite, epipodites and gills. All
these organs are enclosed within a special chamber on each side of the
cephalothorax, which is called gillchamber. The chamber is covered by the
lateral extension of carapace called gillcover or brachiostegite. Each gill
chamber is thus opened ventrally, anteriorly and posteriorly,
A. Lining of the Branchiostegite:
The richly vascularised membrane of
the branchiostegite serves as respiratory surface through which gascous
exchange takes place.
B. Expipodites:
There are small highly vasccularised leaflife membranous
structures, one on the coxal segment of cach maxillipid. These epipodite being
present in the anterior part of the gill-chamber carry out respiratory function
C. Gills:
Among the three sets of respiratory organs, the gills are regarded
as primary respiratory organs. On cach lateral side of the cephalothorax and
beneath the branchiostegites, there are eight gills each attached with the thoracic
wall by a gill root. Seven of these cight gills are scrially arranged, while the
eighth gill is concealed under the second gill.
The gills are crescent-shaped and their sizes increase gradually from
anterior to posterior direction. Each gill consists of a slender axis or base on
which double rows of homboidal leaf-like gill-plates are arranged like the page
of a book.
According to their position and mode of attachment, the gills are of three
types
i) Polobranch attached with the coxa of the around maxillipid.
(1i) Arthrobranch attached with the anthroidal membrane of third
maxillipid.
(iii) Pleurobranch attached with the outer border of the thorax and over
the articulating surface of the walking legs. In prawn. the first gill is podobranch,
second and eight gills are arthrobranches and remaining five gills are
pleurobranches.
Affarent Branchial ⟶ Transverse ➝ Lateral Longitudinal
Channels Channels Channels
⬇️
Marginal Channels
⬇️
Heart ⇦ Efferent Branchial ⇦ Median Longitudinal
Channels Channels
Histological structure of the gill shows that gill base has following layers
the outermost cuticle, inner epidermis and innermost conjective tissues. Each
gill plate is formed by monolayer of cell, sandwitched between two layers of
cuticle. The cellular layer includes two alternately arranged cell types
pigmented and transparent.
Two lateral and one median longitudinal blood channels pass throughout
the length of gill base. The two lateral channels are inter connected by numerous
transverse channels. From cach lateral channel a slender marginal channel is
given to each plate. After covering the entire margin of the plate, the marginal th
channel opens within the medias channel. The gill receives deoxygeneted blood
through afferent branchial channels. Each branch of afferent channel opens
within the transverse channels. From transverse channels the blood passes to
the lateral longitudinal channels and is distributed subsequently within the
gillplates through the marginal channel. After oxidation, the blood from marginal
channel returns to the median channel and then to the efferent branchial vessels,
which convey it to the heart. The course of circulation of blood through the
gills is given below:
Mechanism of Respiration ;
The seaphognathites of maxillae and exopodites of maxillipeds are
responsible for forcing the water to rush inside the gill chamber through posterior
and lateral sides. This water passes out through the anterior end.
During the flow of water the vascularised surface of the branchiostegites
gills and endopodites arc bathed and gaseous exchange occurs through these
areas when dissolved oxygen is taken in and Carbondioxide passess from the
body to the exterior.
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