A study of the blood pathways within the gills of Latimeria has been carried out using light and transmission electron microscopy. Clear evidence has been found for the presence of a secondary non-respiratory circulation in addition to the well-established respiratory pathway through the gill lamellae. All essential components of this system have been observed and have the same relationships and basic structure as comparable secondary systems in actinopterygian and elasmobranch fishes. These include a central venous sinus (CVS), arterio-venous anastomoses (AVAs) and central filament arteries (CFAs). AVAs connect both arterial vessels of the primary circulation and CFAs of the secondary circulation to the CVS. The latter contained many red blood cells. The presence of this secondary circulation in Latimeria gills contrasts with the situation in the gills of the three Living genera of lungfishes where a system possessing the essential features of the tetrapod lymphatic vessel system has been recognized, No suggestions of a true lymphatic vessel system were observed in Latimeria. Other features of gill and vascular anatomy in Latimeria show its closer relationship to dipnoans than other groups of living fishes but evidence derived from this study of the secondary circulation clearly supports the view that the Dipnoi rather than Latimeria represent the Living fishes most closely related to the tetrapods. [References: 60]
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