Spectra of the ¹³CO emission along the arc structures. The positions A–H, where the spectra were extracted relative to HL Tau, are labeled in the upper left corners in the panels in units of arcseconds, and they are also labeled in Fig. 3. Vertical dotted lines denote the systemic velocity measured from the disk rotation. Vertical solid lines present the expected line-of-sight velocities computed from the model of the three-dimensional infalling and rotational motions (Sect. 4.3).