A Cineradiographic Study of Respiration in Myxine Glutinosa L

Abstract

ABSTRACT The anatomy of the respiratory organs, as well as the mechanism of breathing in myxinoids, is distinctly different from that in all other fish-type craniotes even including the petromyzonts. In Myxine the branchial gill lamellae are enclosed in rounded sac-like pouches, with ducts conveying the water from the oesophagus to the gills from which it travels through efferent ducts uniting on each side, finally leaving the animal through paired branchial apertures. In addition to the paired gill pouches there exists an oesophago-cutaneous duct connecting the pharynx to the exterior behind the left, last gill sac. The exterior opening is confluent with the left gill pore. Regarding the course of the respiratory current in this system contradictory explanations have been presented. More recent authors seem, however, to agree upon the concept that the water current is solely dependent upon the pulsating activity of the velum and is not influenced actively by either the gill pouches or their ducts (Gustafson, 1935; Strahan, 1958; Marinelli, 1956). The velum and its connected muscles, which is a unique structure in myxinoids, was already anatomically described by Müller in his Myxine monograph (1835) and later by Fürbringer (1875) and Cole (1907). The first two authors, however, did not contemplate the velum as being of any functional value for respiration. Cole, on the other hand, suggested a role for this structure in the maintenance of the respiratory current. Goodrich (1930) has advocated that breathing in Myxine takes place by expansion and contraction of the muscular elements in the gill sacs. That the gill sacs in Myxine contain distinctly striated muscular elements has been described by Cole (1912) and later by Goodrich (1930) and Hofbauer (1937). It seems obvious, upon reviewing the literature, that the physiological interpretations of this problem in earlier works are mostly founded on anatomical studies with little or no physiological experimentation. The excellent works of Gustafson (1935) and Strahan (1958) base their functional interpretations entirely upon external observations of the living animal or upon dropping coloured solution in the respiratory water and following its course into the nostril and out of the branchial apertures. The present study attempts to analyse the mechanics of respiration in Myxine by utilizing modern cineradiographic equipment which allows fluoroscopic examination of all events inside the respiratory passages; these may be recorded on a camera at 26 frames per second. The method has been used earlier for the study of the circulation in snakes (Johansen & Hol, 1960) and of the central circulation in Myxine (Hol & Johansen, 1960).