metamorphosed adult (b) and free-swimming larvae (c)

Larval sea-quirts (Tunicata) [right] possess the four major features of modern living chordates, pharyngeal gill slits, a dorsal hollow nerve chord supported by a stiff notochord so as to provide a tail that allows the larva to move freely. During maturation, the motile larvae settles against a hard substrate, and the adult resorbs both nerve chord and notochord. The gill slits metamorphose into a seining basket to catch food particles. Retention of the nerve cord, notochord, and tail into a sexually-mature, motile adult is one proposed mechanism for the evolution of Chordata.

Biologist and poet Andrew Lang (1844 - 1912) reflects on the implications of this for its close relatives, Us.

Figure ©1999 Campbell et al.;  text © 2020 by Steven M. Carr 


"The Ancestor remote of Man,"
Says Darwin, "is th' Ascidian,"
A scanty sort of water-beast
That, ninety million years at least
Before Gorillas came to be,
Went swimming up and down the sea.

Their ancestors the pious praise,
And like to imitate their ways;
How, then, does our first parent live,
What lesson has his life to give?

Th' Ascidian tadpole, young and gay,
Doth Life with one bright eye survey,
His consciousness has easy play.
He's sensitive to grief and pain,
Has tail, and spine, and bears a brain,
And everything that fits the state
Of creatures we call vertebrate.

But age comes on; with sudden shock
He sticks his head against a rock!
His tail drops off, his eye drops in,
His brain's absorbed into his skin;
He does not move, nor feel, nor know
The tidal water's ebb and flow,
But still abides, unstirred, alone,
A sucker sticking to a stone.

And we, his children, truly we
In youth are, like the Tadpole, free.
And where we would we blithely go,
Have brains and hearts, and feel and know.
Then Age comes on! To Habit we
Affix ourselves and are not free;
Th' Ascidian's rooted to a rock,
And we are bond-slaves of the clock;
Our rocks are Medicine--Letters--Law,
From these our heads we cannot draw:
Our loves drop off, our hearts drop in,
And daily thicker grows our skin.

Ah, scarce we live, we scarcely know
The wide world's moving ebb and flow,
The clanging currents ring and shock,
But we are rooted to the rock.
And thus at ending of his span,
Blind, deaf, and indolent, does Man
Revert to the Ascidian.

                    -Andrew Lang (1844 - 1912)