Embodiments of the present disclosure are directed to apparatuses, systems, and methods for merging an endoscopic basket delivery catheter onto a fixed guidewire. In one implementation, a catheter may include a sheath having a saddle secured to a distal portion thereof. The sheath may constrain an endoscopic tool such as a basket therein, while the saddle may receive a portion of a guidewire. The saddle may include a slit extending along its longitudinal length. The slit may be widened by a working member of an adapter to allow passage of a locked guidewire into the guidewire lumen of the saddle. The catheter may be merged onto the guidewire via the slit and delivered to the desired treatment device without unlocking of the guidewire. Advantageously, access to at least one desired treatment site may be maintained with the guidewire during merging of the catheter.