Morphological studies on envelopes of oncospheres of the hymenolepidid cestode, Fimbriaria fasciolaris [Pallas, 1781]

The envelopes of oncospheres of Fimbriaria fasciolaris, found in the distal part of the strobila or free, were the subject of scanning electron microscope (SEM) and light microscope (LM) studies. The oncospheres inside the strobila were in close apposition to the uterine wall and showed morphological ties suggesting metabolic interactions. SEM studies allowed us to discern 3 stages of uterine development: early - with a continuous, tubular, and branched uterus; intermediate - with bulging parts of the uterus forming uterine capsules packed with oncospheres; late - with the uterus discontinuous, breaking down into uterine capsules, either individual or connected into chains of different lengths, containing 1 to 12 oncospheres. The uterine epithelium within uterine capsules was structurally heterogeneous, closely connected with the oncospheres, constituting a common uterine envelope. Infective eggs outside the strobila were deprived of the uterine envelope, and were joined together by separate external envelopes, easily visible in the LM. Live oncospheres observed over a 24 h period after liberation from the strobila exhibited alterations in taxonomically important features, such as dimensions and shape of the external envelope. The possible roles of different envelopes are discussed.