This chapter discusses four cyprinodont fishes, all of which inhabit surface to near-surface zones of the water body. It evaluates the nature of the relationship between the stratum of water body and morphological specializations of the canal system. Fishes and aquatic amphibians are characterized by a unique surface receptor system, the lateral line system. The lateral line system occurs symmetrically on both sides of the head and trunk. In the head region of fishes the lateral line system is conspicuous and elaborate and referred to as the cephalic sensory canal system whereas in the trunk and tail regions it forms the lateral line proper. Cyprinodont fishes are small in size and essentially surface dwellers. Their thin and light skull probably cannot afford more ossification to cover the sensory canal. Therefore, absence of ossification over the cephalic sensory canals of cyprinodonts may be considered an adaptive feature.