Description of musculature differences in spargana of Spirometra (Cestoda; Pseudophyllidea) and tetrathyridia of Mesocestoides (Cestoda; Cyclophyllidea) and their value in identification.

About two years ago our laboratory received for identification a sparganum of Spirometra sp. (about 10 cm long) removed from a nodule in the knee of a Tanzanian negro by Doctor Knut Saug. About the same time we received for identification two tetrathyridia of Mesocestoides sp. (about 8 cm long) found in the peritoneum of a Norwegian cat. The latter was the first record of tetrathyridia found in domestic animals in Norway (BERG & ANDERSEN, 1982). Macroscopically these two larval types were extremely similar and impossible to separate. Both larval types have an anterior end, a bit broader than the rest of the larval body, where the invaginated scolex is situated. In transverse sections of the region, the sparganum showed bothrial slits (Fig. 1), while the tetrathyridia showed typical suckers (Fig. 2). Transverse sections of the larval body also showed very distinct differences in the musculature between these two larval types (Figs. 3 & 4). In spargana of Spirometra spp. (this was also verified by sectioning S. mansonoides larvae from experimentally infected white mice) the parenchymal longitudinal musculature is arranged in distinct bundles scattered more or less randomly throughout the parenchyma (Fig. 3), while in tetrathyridia of Mesocestoides the parenchymal longitudinal musculature forms an elliptic band. On the dorsal and ventral side of the larval body of Mesocestoides there are muscle bundles consisting of several muscle fibres; moving laterally, these bundles decrease in size until only single muscle fibres are seen (Fig. 4). The macroscopical similarity between spargana of Spirometra and tetrathyridia of some Mesocestoides species has been pointed out earlier and has, in previous years, lead to some confusion and misidentification (see NELSON et al., 1965). In their paper on the significance of wild animals in the transmission of cestodes of medical importance in Kenya, NELSON et al. (1965) state that the identification and separation of tetrathyridia and spargana, without histological sectioning of the scolex end of the larvae, is unreliable. They further state that all "spargana" so far collected from animals in Kenya on sectioning of the scolex end of the larvae proved to be tetrathyridia, the larval stage of Mesocestoides. The musculature has not, to my knowledge, previously been properly described for either of these larval types, although TAYLOR (1976) shows a picture of a musculature very similar to that illustrated in this paper from a human case of sparganosis in the U.S.A. In the past, there has been some argument about the value or otherwise of deepseated and less variable systems of organs, such as the genitalia or muscles, in identification of tapeworms. SOUTHWELL (1929) argued strongly in favour of the major value of external metromorphic characters, while WOODLAND (1925 and 1927) was of