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Based on ca 100 lacustrine cichlid species and following a holistic procedure of functional 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 with the oral jaws are involved and (2) those in which the food is directly sucked into the buccal cavity 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 (comparisons in a constructional context), and how these spatial demands 'affect' the function 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 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 apparatus) 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 particular 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 structure of the feeding apparatus are discussed.
C.D.N. Barel (Fri,) studied this question.