Sinuses are devoid of an epithelial lining and thus “esemble the haemocoel of arthropods and molluscs.
Main haemal sinuses are as follows-
(1) Oral haemal ring:
It is the circular haemal sinus, located around the mouth just below the ring canal of the water vascular system.
(2) Radial haemal sinuses or strands:
These arise radially from the oral haemal ring and each one extends into each arm. They send out short branches to the tube feet.
(3) Axial gland:
This is the main part of the haemal system often referred to as the heart or brown gland. It remains enclosed within the axial sinus of perihaemal system.
It contains several inter communicating small spaces filled with a fluid and coelomocytes. Axial gland is connected with the oral and aboral haemal sinuses at its oral and aboral ends respectively. It gives off several branches to the various parts of water vascular system and alimentary canal.
(4) Aboral haemal ring:
It is a pentagonal ring canal lying beneath the aboral surface of the central disc. From this canal extend five pairs of genital haemal strands to the gonads.
Haemal system acts as a pathway for the distribution of food substances carried by the coelomocytes.
Flow of fluid is maintained by the contraction of dorsal sac. Axial glands act as a genital stolon producing sex cells, which reach the gonads through aboral haemal ring and its branches.
(ii) Perihaemal system:
It is derived from the coelom and consists of a system of tubular sinuses or vessels, which enclose the sinuses of the haemal system.
1. Oral perihaemal orhyponeural ring sinus:
It encloses the oral haemal ring sinus.
2. Radial perihaemal sinuses or canals:
Each canal encloses a radial haemal strand running into an arm. Haemal strand divides perihaemal sinus into two lateral longitudinal sinuses.
1. Axial sinuses:
They enclose the axial gland of haemal system and stone canal of water vascular system. All the three axial sinuses form together the axial complex
2. Aboral perihaemal ring sinus:
It encloses the aboral haemal ring sinus.
3. Genital perihaemal sinuses:
It encloses the gonads and the genital haemal strands.
4. Marginal sinuses:
These are 10 in number two in each arm, one on each side below the marginal nerve cord. They open into the radial perihaemal sinuses of their respective arms.
7. Peribranchial sinuses:
These are circular sinuses around the basal parts of the papulae.