Evolution What the Fossils Say and Why it Matters

Fish Tales 215

(fig. 9.6A and B) burrow tail-first into the sandy sea bottom and then use the tentacles around
the mouth and pharynx to filter feed. But the anatomy of these animals bears a remarkable
resemblance to primitive fish. Lancelets have a well-defined notochord, a nerve cord along
the back, a digestive tract along the belly, and many V-shaped myomeres down the length
of the body. Their pharyngeal basket is well developed, with over a 100 “gill slits” like those
of a primitive fish. In addition, they are the most primitive chordates to have a liver and a

FIGURE 9.6. (A) The lancelets, or amphioxus (Branchiostoma) are the most fishlike and specialized of the
nonvertebrate chordates. They have a long eellike body with muscles running down the entire length and
a notochord supporting the entire body, but the mouth is still a simple filter-feeding pharynx. In life (top
diagram) they embed themselves in the sediment with their heads protruding, catching tiny food particles with
their mouth filter feeding in the current. (After Barnes 1986, drawn by Carl Buell) (B) Photograph of the living
lancelet. (From IMSI Photo Images, Inc.) (C) The middle Cambrian Burgess Shale fossil lancelet Pikaia. (D) The
Early Cambrian Chengjiang lancelet fossil known as Yunnanozoon. ([C and D] photos courtesy D. Briggs)

Muddy sediment

Dorsal nerve cord

Notochord Diverticulum Myotomes

Pharynx

Buccal cirri

Endostyle

Gonads

Atriopore

Intestine

Fin-ray boxes Anus

Sand

10 mm

(A)

(B) (C)

(D)

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