Evolution What the Fossils Say and Why it Matters

FIGURE 9.3. The hemichordates include the pterobranchs and the acorn worm (shown here). This creature
looks superficially wormlike but has the chordate feature of a pharynx with gill slits, and the precursors of a
notochord, as well as a dorsal nerve cord and ventral digestive tract (the reverse of all other true “worms”).
Behind the acorn worm is a sketch of its burrow, and the castings that it leaves outside the tail end of the
burrow. (Part (A) redrawn after Barnes 1986; (B) courtesy Wikimedia Commons)

(A)

Collar

Proboscis

Mouth

Gonad

Gill slits

Anus

5 mm

(B)