This larva is adapted for swimming, which is accomplished by means of the velum.
In Fig. 121 I give a picture of this surface-marking of the velum.
The velum has the normal situation and relation to its mouth.
There is a veliger stage in development, but the velum is reduced.
This prolongation forms the anterior part of the œsophagus of the adult, and joins the primitive oral cavity at the velum.
Such organs form projections on the upper surface of the velum.
It is placed in the anterior region of the body, and opens ventrally on each side, a short way behind the velum.
Physiologically the velum is frequently important from the plexus of blood-vessels which passes with it into the III.
After a little while try to move the velum, closing and opening the nose passage, without uttering a sound and without breathing.
Still watching your mouth, inhale through the nose and exhale through the mouth; see how the velum moves as you do this.