Mechanisms of Sound Production in Delphinid Cetaceans: A Review and some Anatomical Considerations

The past literature describing the possible sites of the sound-producingmechanisms in delphinid cetaceans is reviewed. The morphologyof the nasal sac system of delphinids which has been implicatedin the production of sounds, by most investigations, is discussedwith special emphasis placed on the physical characteristicsof these sounds. New data on the histological structure of theepithelia throughout the nasal region of a delphinid are presentedwith some suggestions as to its function. The presence and structureof glandular tissues are described along with a discussion oftheir potential role in the production of sound. It is concludedthat the theories implicating the nasal sac systems of odontocetecetaceans in the production of sound are additionally supportedby certain anatomical specializations adjacent to the tissuesof this system.     

Sound Producing Mechanisms in Recent Reptiles: Review and Comment

Reptiles produce sound by three categories of devices: thoseinvolving massive air expulsions or movements, those involvingmodulations of intermittent air movements through a modifiedglottis, and those involving rubbing or vibration of the integument.Each ocours in numerous species and there is good evidence ofmultiple origins. Most of the devices are used as predator deterrents,but there are a few cases of sound being used as an intraspecificcommunicating device. The various structures are here discussed.One may ask the general question whether the distribution ofreptilian sensory modalities may not have forced this rathermiscellaneous distribution of signalling devices.     

Embryonic Tissue Interactions as the Basis for Morphological Change in Evolution

Phenotypic changes over geological time must result from alterationsin the morphogenetic mechanisms associated with organogenesis.Recent advances in our understanding of such mechanisms suggestthat there are certain patterns of metabolic activity and organellesynthesis which are present in many different cell lineagesat different times during embryonic development, and that thisubiquity of genomic potential is available for expression atany time. Microevolutionary sequences of morphological changeprobably result from modulations of quantitative aspects oforganogenesis, e.g., rates of cell proliferation, or cell density.It is also possible that certain macroevolutionary steps ("neomorphs")may result from qualitative changes in the developmental programs,and such events underly "ultimate refinement" of morphologicaladaptation. The selection pressures involved in these differentsequences of morphological change "see" embryonic processesin different ways. This thesis is illustrated with referenceto the evolutionary history of the tetrapod limb.     

The Structure and Permeability of Integument

SYNOPSIS. The skin is a heterogeneous, multimembrane systemwith multiple diffusion pathways and potentially numerous rate-limitingbarriers for specific molecules that are exchanged with theenvironment. A broad survey of animals indicates that integumentarycoverings are morphologically, biochemically and embryologicallydiverse but with common themes of adaptation. The evolutionaryproliferation of intercellular junctions, fibrous or mineralizedprotective barriers, and lipoid waterproofing barriers emphasizethe generality of diffusion limitations. However, the regulationof specific exchange processes such as the flux rates of respiratorygases entails a complex interaction of multiple factors affectingboth diffusion and perfusion limitations. Consideration of specificpathways and rate limitations for diffusion of various substancessuggests that the permeation of skin by specific molecules canbe partially independent of other exchange processes. Our understandingof regulated permeability is, however, lacking in mechanisticand integrated analysis in most cases. Comprehensive understandingof the integument as a regulatory pathway of communication withthe environment will require comparative studies of specifictransport pathways, their rate-limiting resistances, and theinteractions as well as individual regulation of transportedmolecules.