NON-RESPIRATORY BLOOD-VESSELS IN LATIMERIA GILL FILAMENTS

A study of the blood pathways within the gills of Latimeria has been c arried out using light and transmission electron microscopy. Clear evi dence has been found for the presence of a secondary non-respiratory c irculation in addition to the well-established respiratory pathway thr ough the gill lamellae. All essential components of this system have b een observed and have the same relationships and basic structure as co mparable secondary systems in actinopterygian and elasmobranch fishes. These include a central venous sinus (CVS), arterio-venous anastomose s (AVAs) and central filament arteries (CFAs). AVAs connect both arter ial vessels of the primary circulation and CFAs of the secondary circu lation to the CVS. The latter contained many red blood cells. The pres ence of this secondary circulation in Latimeria gills contrasts with t he situation in the gills of the three Living genera of lungfishes whe re a system possessing the essential features of the tetrapod lymphati c vessel system has been recognized, No suggestions of a true lymphati c vessel system were observed in Latimeria. Other features of gill and vascular anatomy in Latimeria show its closer relationship to dipnoan s than other groups of living fishes but evidence derived from this st udy of the secondary circulation clearly supports the view that the Di pnoi rather than Latimeria represent the Living fishes most closely re lated to the tetrapods.