Based on ca 100 lacustrine cichlid species and following a holistic procedure of func- tional morphology, this paper analyses what constraints spatial relations of structures may set on the compatibility of functions. Among the many feeding behaviours of cichlids two groups are distinguished: (1) those in which powerful biting with the oral jaws are involved and (2) those in which the food is directly sucked into the buccal cavi- ty without prior manipulation by the oral jaws. Related to the core functions "biting" and "sucking only" two types of oral jaw apparatuses (OJA) are distinguished and compared (i. e. comparison in a functional context). Taking the head as the available space (constructional component) it is subsequently investigated what spatial demands there are to accommodate these two types of OJA together with other apparatuses (com- parisons in a constructional context), and how these spatial demands 'affect' the func- tion of the other apparatuses, especially the expansion apparatus (EA) and outer head shape (OHS, part of the locomotory apparatus). It is demonstrated that the suction feeding and biting can be combined to a certain extent and at certain costs. Increase in streamline is inversely related to active head expansion, and in biters is also inversely related to the size of the maximally expanded buccal cavity. The possible implication for the tripartite compatability of (1) the various types of locomotion, (2) suction feeding and (3) feeding involving oral manipulation of the food, are discussed. For a number of other apparatuses (e.g. gill-apparatus and pharyngeal jaw ap- paratus) similar but less extensive analyses of spatial relations are made. Among these, examples are given of diversity in spatial relations: e.g. the relations between a par- ticular outer head shape and differing head structures (see the section on 'inner ear'). The complex and indirect relations between the food-category eaten and the struc- ture of the feeding apparatus are discussed.