The Morphology and Development of Obelidium Mucronatum

SUMMARY

In the foregoing paper an account of the morphology and development of Obelidium mucronatum, a seemingly rare chytrid inhabiting the submerged exuviae of midges and caddisflies in the vicinity of Ann Arbor, Michigan, is given. The zoöspore after becoming quiescent and retracting its cilium produces an acumination which will form the apical spine of the mature sporangium. Coincident with this or generally later, the rudiments of the rhizoidal system are produced from the opposite part of the spore body. Subsequently, an apophysis is formed by the inflation of the primary germ tube and portions of the primary branches. The sporangium develops from the expanded body of the original spore and at maturity consists of three parts, the solid apical spine, the narrowly to broadly ovate mid-region which contains the bulk of the protoplasm carried into it from the rhizoids, and a lowermost, cup-like, distinctly thick-walled region which may be prolonged into a funnel- or stalk-like structure. The mature sporangium is separated from the profusely branched and extensive rhizoidal system by a septum laid down between the base of the sporangium and the sub-sporangial apophysis. The posteriorly uniciliate zoöspores are delimited within the sporangium, emerge en masse through a sub-apical pore, and after their discharge rest for a time at the orifice before undergoing a period of preparatory swarming which terminates with their dispersal. Sometimes, instead of a Rhizidium-like method of development, O. mucronatum may exhibit a Chytridium habit of growth, a part of the thallus being extramatrical and the remainder intramatrical. No resting spores have been found. Similarity in body structure and method of development of Obelidium, Rhizoclosmatium, Siphonaria, and Asterophlyctis, all exuviae-inhabiting forms, suggests that they are closely related genera.