Mesonephric duct

A mesonephric duct is a collecting duct with a relatively wide lumen and thick epithelium.

... a single large convoluted collecting duct with a wide lumen and thick pseudostratified epithelium, the mesonephric duct, which constituted the sole structure of the caudal kidney [1].
The killifish kidney is so small that the mesonephric duct is also included in the ultrathin sections. The fine structure of this duct is similar to that of the collecting tubule, although its diameter is larger than that of the collecting tubule. The duct is also invested by a relatively thick layer of smooth muscle cells, which appear to be mainly obliquely arranged [2].

References

Prodocimo & Freire (2003). Glomeruli and renal tubules are restricted to the cranial kidney of the adult estuarine Sphoeroides testudineus. J Fish Biol 63, 1258-1265.
Tsuneki et al. (1984). Electron-microscopic study of innervation of smooth muscle cells surrounding collecting tubules of the fish kidney. Cell Tissue Res 238, 307-312.