In prawn an “open type” or lacunar type of blood vascular system is found. This type of system is characterized by the fact that blood circulates through open spaces rather than capillaries.
Capillaries are absent and open spaces are called lacunae or sinuses. It includes the following organs.
Image Source: 185.26.182.219
ADVERTISEMENTS:
1. Pericardium:
It is a thin walled spaceous haemocoelomic chamber containing the heart, lying above the hepato pancreas and gonads.
The floor of the pericardium is in the form of a thin horizontal septum, which is attached in front and behind to the dorsal body wall and on the lateral sides to the thoracic wall.
2. Heart:
ADVERTISEMENTS:
The heart is a triangular muscular structure suspended in the pericardium by three muscle strands.
A median longitudinal cardiopyloric strand extends from the apex of the pyloric stomach. The two lateral strands extend from the lateral angels of the heart to the body wall. The body of heart is perforated by five pairs of slit-like apertures calied ostia.
The first pair of ostia is situated mid-dorsally, 2nd pair midventrally, 3rd pair posteriorly, 4th pair antero-laterally and the 5th postero-laterally. Blood from pericardial sinus enters the heart through these ostia.
In a section, cavity of heart appears sponge-like, filled with numerous interlacing muscle fibers and blood in the interspaces.
ADVERTISEMENTS:
3. Arteries:
Arteries are thick, strong and muscular tubes arise from the heart and supply blood in different parts of the body. Principal arteries are the following:
(a) Median opthalmic artery:
It is a single median cephalic or opthalmic artery arising from the apex of the heart. It runs forward, mid-dorsally along the renal sac to supply blood to the oesophagus, cardiac stomach and head. It joins the two antennary arteries above oesophagus.
(b) Antennary arteries:
A pair of lateral cephalic or antennary arteries arises from the apex of the heart, one on each outer side of median opthalmic artery.
Each artery runs forward obliquely and is branched into different arteries. A pericardial branch to cardiac stomach and a mandibular branch to mandibular muscle.
Then it bifurcates into a dorsal and a ventral branch. The ventral branch further divides to supply the antennule, antenna and renal organ.
The dorsal branch gives off two sub branches, the outer one is, optic artery supplies to the eyes and the inner artery which meets its fellow of the opposite side and the median opthalimic artery forming circulus cephalicus. A pair of rostral arteries arise from the circulus cephalicus to go to rostrum.
(c) Hepatic arteries:
These arteries arise from ventrolateral sides of the heart, one on each side just behind the antennary artery. Each ramifies into capillaries in the hepatopancreas.
(d) Median Posterior Artery:
A short, stout median artery arises from the postero-ventral side of the heart. Immediately it bifurcates into two arteries.
(i) Supraintestinal or dorsal abdominal artery:
It runs backward along the dorsal surface of the intestine upto the hind gut. It supplies blood to intestine and abdominal muscles.
(ii) The sternal artery:
It runs obliquely downwards and passes through an aperture in the ventral thoracic ganglionic mass.
On reaching the ventral side it bifurcates into two (a) the ventral thoracic artery runs forward and supplies blood to the sternal region of the thorax, (b) The ventral abdominal artery runs backwards and supplies blood to the ventral abdominal region.
4. Blood lacunae:
Main arteries don’t terminate into fine capillaries in various organs; instead they open into blood lacunae of the haemocoel.
As there are no veins in the prawn, these lacunae join one another and form a pair of large ill- defined ventral sinuses, situated below the hepatopancreas and muscles of the thorax. The two ventral sinuses communicate with each other at various places.
5. Blood channels:
From ventral sinuses, blood is carried out to the gills on each side through six afferent branchial channels.
The channels are lacunar tubes without proper walls. The sinuses and channels comprise the open portion of circulatory system. From the gills the oxygenated blood is carried to the pericardium by six efferent branchial channels.
Blood and its course of circulation:
Blood is a colourless thin fluid containing white corpuscles or leucocytes and a dissolved respiratory pigment haemocyanin.
The metal component of haemocyanin is copper and it turns blue when it combines with oxygen. Blood of prawn shows clotting properties.
The circulation of blood starts from the pumping action of the heart. The heart by means of its rhythmic contraction pumps blood into the arteries, which distribute it to all parts of the body. After passing through gills and different channels blood returns back into the heart.