The structure of cordons is a principal characteristic in the identification of acuariid nematodes. The cordons of Desportesius invaginatus are formed of consecutive structurally elaborated plates of variable size and topography. Energy dispersive X-ray microanalysis (EDXMA) is an analytical technique that has been used to evaluate the element composition and crystalline nature of the body surface of cestodes, nematodes and acanthocephalans. It has been also used to measure the concentration of

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... minor elements [sulphur (S), phosphorus (P), calcium (Ca) and potassium (K)] and trace elements (Fe, Zn). Objective: The present study was undertaken to investigate the concentration and distribution pattern of some chemical elements, namely: S, Ca and P in the cuticular elaborations of the cordon plates of D. invaginatus using dispersive X-ray spectrometry, aiming to supplement information regarding the composition and function of these structures. Results: Energy dispersive X-ray microanalysis revealed variations in the concentration of Ca, S and P in different regions of the cuticular ridges (tip, middle and base) of the three regions (anterior, middle, posterior) of the cordons of D. invaginatus. In the anterior region of the cordon, P was the highest in concentration followed by Ca then S; in the middle region, S exhibited highest concentration followed by P then Ca; while in the posterior region, S showed the highest concentration followed by Ca and P. The study revealed a specific pattern for the distribution and concentration of each element in relation to the region in the cuticular ridges of the cordons. Highest concentration of Ca and S was presented in the posterior region, where Ca decreased significantly towards the middle region to reach the lowest levels, then significantly increased again towards the anterior region; while S reached its lowest concentration in the anterior region, where P recorded its highest concentration. Conclusion: The concentration and distribution pattern of S, Ca, and P could be related to the anchorage function of the ridges. The high concentration of S in the base gives flexibility to these structures and facilitates the piercing action of the host tissue; while the accumulation of both Ca and P at the tip could result from the polymerization of both elements to give the rigidity and hardness that secure the position of the parasite in the host's tissue and prevent its detachment.