Some anatomical structures vary greatly in number among species, a phenomenon that often remains unexplained. We investigate interspecific variation in the number of collar spines among trematodes from the superfamily Echinostomatoidea, using a dataset comprising hundreds of species. These trematodes possess a ring of spines around their anterior sucker; in some families, they form 2 arcs on either side of the sucker, with a central gap, whereas in other families, they form a continuous collar with no gap. First, we confirm that even numbers of spines are the norm among species in which they are arranged as 2 arcs with a central gap, while odd numbers (mainly prime numbers) predominate among species in which spines form a continuous collar. Second, we tested whether variation among species in the number of spines might reflect selective pressures. The spines serve to attach the worm to the inside lining of the host gut. Our analysis confirms that spine numbers correlate positively with worm body size among echinostomes, supporting the hypothesis that larger worms require more spines for stronger attachment. Finally, we tested whether phylogenetic conservatism may explain interspecific variation in the number of collar spines, i.e. whether closely related species have more similar numbers of spines than expected by chance due to shared ancestry. Our analysis confirms that spine numbers show strong phylogenetic conservatism across species. Overall, our findings indicate that the number of collar spines, a hallmark of echinostomes, is the product of conserved phylogenetic inheritance overlaid by adaptive functional adjustments.