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| Blood Vascular system of a cockroach |
By:
Snigdha |
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The circulatory system is poorly developed. It is of the lacunar or open type, there being no capillaries or veins. It consists of the heart, a blood vessel called the aorta and the haemocoel.
The heart is a long, contractile, narrow tube, lying dorsally in the mid-line, just beneath the terga of the thorax and abdomen. It is enclosed in a pericardial sinus, which is separated from the general haemocoel by a thin hcrizontal pericardial septum. The heart consists of thirteen funnel-shaped and segmentally arranged chambers, each communicating by a valvular opening with the one in front
of it. The hinder end of each chamber has a pair of minute, lateral and valvular openings, the ostia, through which the blood enters from the pericardium. The arrangement of valves prevents the backward flow of blood through the ostia and permits only forward flow inside the heart.
The heart is closed behind but is continued forwards as a short and narrow tube without ostia, the aorta, which opens into the haemocoel in the head.
Running from the perjcardial wall to the terga is a series of fan-shaped and paired alary muscles. The contraction of these muscles enlarges the pericardial space so that blood flows into it from the underlying perivisceral sinus. When the alary muscles relax, the blood is forced through the ostia into the heart. The heart and the aorta contract peristaltically from behind forwards, driving the blood anteriorly into the head. It then passes back from the haemocoel of the head into that of the thorax and abdomen and enters the pericardium.
The blood of cockroach consists of plasma and of colourless cells, the leucocytes. It differs from the blood of earthworm and vertebrates in being colour less; for, it lacks the respiratory pigment, haemoglobin and does not take part in respiration, which is performed by the highly developed tracheal system. The blood serves mainly as the distributing and collecting medium for food and wastes. During circulation, the blood comes in direct contact with the tissues. This is not so in earthworm and vertebrates, in which the blood remains inside the blood vessels and the tissues are bathed in the lymph. The “blood” of cockroach and. other insects is thus both blood and lymph and is therefore called haernolymp.The blood also serves as a reservoir for food; and when under pressure, it helps in batching from the egg, in moulting and in the expansion of wings. Besides, it contains some amoeboid cells, the phagocytes, which destroy dangerous bacteria and other harmful parasites, etc.
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