The right and left halves are completely divided by septa, no mixture of the venous and arterial blood being possible, an advance upon reptilian conditions, even the highest.
Thus the body cavity is divided into a sequence of chambers by transverse septa; and even among the Hirudinea, Spadella cephaloptera (Busch).
St, Septa dividing body-cavity transversely.
This is the typical arrangement, which is exhibited in the majority of the Polychaeta and Oligochaeta; in these the successive chambers of the coelom are separated by the intersegmental septa, sheets of muscle fibres extending from the body wall to the gut and thus forming partitions across the body.
The successive cavities are not, however, completely closed from each other; there is some communication between adjoining segments, and the septa are sometimes deficient here and there.