The sinus system of hagfishes - Lymphatic or secondary circulatory system?

Apart from the vascular system of arteries, capillaries and veins, hagfish possess an extensive system of vessels and sinuses originally believed to be lymphatic. The occurrence of red cells in parts of the system was considered to be an artefact. Later, connections were found between certain sinuses and arteries. These sinuses were termed 'red lymphatics'. Other parts of the system were termed 'white' lymphatics and believed to be true lymphatics. New observations show that even 'white' lymphatics communicate with the arterial system. It is concluded that the system is comparable to the secondary vascular system of teleosts. A new system of fine channels surrounded by outgrowths from mesothelial cells of the serosal surface of the intestine is described. These channels communicate with secondary vessels of the intestine. Finally characteristic crypts in the serosal surface of the intestine are described.