In all organisms, messenger-directed protein synthesis is catalyzed by ribonucleoprotein particles called ribosomes. A ribosome is typically composed of one small and one large subunit which contain one short (18S) and one long (28S) rRNAs, respectively. Surprisingly, in this study, three similar size rRNAs (18-21S) were revealed in the electrophoresis profile of the total RNAs of tapeworm _Spirometra erinaceiuropaei_. Northern blot analysis shows that one of the three bands belongs to 18S rRNA, and the other two bands are of 28S rRNAs, implying structurally distinct ribosomes in this intermediate animal. Furthermore, similar, but not identical profiles were observed in two other tapeworms _Diphyllobothrium hottai_ and _Diphyllobothrium Nipponkaiizeme_. Relevant to this finding, in flatworm _Paragonimus westermani_, 18S rRNAs were found much more numerous than 28S rRNAs. Moreover, consistent with this biochemical finding, transmission electron microscopy examinations show that the ribosomes isolated from _Spirometra erinaceiuropaei_ are composed of either one ball or two similar size subunits (balls), while the structure of ribosomes isolated from control liver tissue exactly match the conventional large and small subunit ribosome model. Our study provides direct biochemical and biophysical evidence of structurally distinct novel type of ribosomes in intermediate eukaryotic flatworms. These finding may be important for re-recognition of biological protein synthesis and evolutionary process of living things
