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Transport in Cnidarians:

Cnidarians such as Hydra have an internal fluid filled gastro vascular cavity. This cavity supplies nutrients for all body cells lining the cavity, provides oxygen from the water in the cavity and is a reservoir for carbon dioxide and other wastes. Simple body movement moves the fluid.
Coelomic fluid:

Some animals like Echinoderms, Annelids and Sipunculans are coelomic fluid as Supplementary or sole circulatory system. Coelomic fluid may be identified in composition to interstitial fluids or may differ particularly with respect to specific proteins and cells. Coelomic fluid transports gases, nutrients and waste products. It also functions in certain invertebrates like Annelids as a hydrostatic skeleton.


It is circulatory fluid of animals with an open circulatory system. Most Arthropods, Ascidians and many Molluscs have hemolymph. In these animals heart pumps hemolymph at low pressure through vessels to tissue spaces (hemocoel) and sinuses. Generally hemolymph volume is high and the circulation slow. In the process of movement, essential gases, nutrients and wastes are transported.

Many times hemolymph has no circulatory functions. For example in insects, hemolymph pressure assists in molting of old cuticle and in inflation of the wings. In certain jumping spiders, hydrostatic pressure of hemolymph provides hydraulic mechanism for limb extension.

Blood Cells:

Coelomic fluid, hemolymph or blood of most animals contains circulating cells called blood cells or hemocytes. Some cells contain respiratory pigment such as haemoglobin and are called erythrocytes. These cells are usually present in high number to facilitate oxygen transport. Cells that do not contain respiratory pigments have other functions such as blood clotting. Types of blood cells vary in different invertebrates.

(1) Annelid blood contains hemocytes that are phagocytic. The coelomic fluid contains a variety of coelomocytes (amoebyocytes, eleocytes, lymphocytes, linocytes) that function in phagocytosis, glycogen storage, encapsulation, defense response and excretion.

(2) Hemolymph of Molluscs has two general types of hemocytes (amoebocytes and gametocytes) that have also nacrezation (pearl formation) in some bivalves.

(3) Insect hemolymph contains large number of various haemocyte types that function in phagocytosis, encapsulation and clotting.

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