The arteries and veins have proper endothelial walls; they pass abruptly into the sinuses and in some cases communication is effected by orifices in the walls of the vessels, as for example in the vena cava of Nautilus.
The external duct of this gland is often dilated into a bladder, and may sometimes send out diverticula, forming a complex system of sinuses ramifying through the body.
(After Kovalevsky.) network places these sinuses and the lateral sinuses in communication.
These sinuses are completely shut off from all other cavities, they do not open into the main coelomic space nor into the small arm-sinus, nor does the right sinus communicate with the left.
There is further a great tendency for the endothelial cells to form muscles, and this is especially pronounced in the small arm-sinus, where a conspicuous muscle is built up. The mantle-sinuses which form the chief spaces in the mantle are diverticula of the main coelomic cavity.