MORPHOLOGICAL FEATURES OF FUNCTIONAL SIGNIFICANCE IN THE GILLS OF THE SPINY DOGFISH,<i>SQUALUS ACANTHIAS</i>

Abstract

1. The course of blood through the gills is described. 2. Blood passes first into arteries which parallel the filaments and lie at their base. 3. Blood then passes into the overlying parallel cavernous bodies through any of the approximately 20 openings per millimeter. 4. The cavernous bodies have non-elastic walls and, passing across the lumen, non-elastic columns. Blood is free to flow in any direction through the cavernous bodies. Each cavernous body has approximately 20 pairs of exits per millimeter, each one opening into a secondary lamella. 5. The secondary lamella does not contain capillaries, but consists of two sheets of epithelium held together, with a constant space between them, by large numbers of pilaster cells. Within the single plane of this structure blood is free to move in a variety of directions between its entrance and its exit into the distal artery. 6. Within the freedom of movement provided by the basal artery, cavernous body and secondary lamella, blood is free to follow the line of least resistance. This results in its following the line of least pressure fall, and accounts for the small loss of pressure during transit of the gill. 7. The cavernous body appears to be rather rigid, and the large number of pilaster cells restricts the separation of the two epithelial layers and must impart a degree of rigidity to the walls of the vascular spaces. It is due to the consequent restriction of stretch and recoil that the pulse wave is relatively undamped and continues into the dorsal aorta. 8. The arrangement of blood vessels into myriads of parallel sheets which start and end abruptly provides the anatomical basis by which large amounts of blood are passed through a seemingly highly restricted area. 9. There is evidence that both the basal and distal arteries have sufficient muscle tissue to be potentially capable of some degree of contractility, a factor which should be taken into consideration relative to vascular changes. 10. There is no apparent explanation for the presence, in the basal half of the water passages of the anterior hemibranch, of an epithelium with what appears to be a highly developed brush border. 11. The anatomical arrangement of the gill seems to fit a multicapillary model better than a counter-current one. However more information is needed as to the details of water flow in relation to vascular structures before a definite conclusion can be reached.