As you will see in the illustration (figs. 1 and 2) of one of these youngsters, the resemblance to the tadpole of the frog is most striking, and in some of the points wherein the sea-squirt differs from the tadpole, it represents a yet earlier structural stage which frogs have long since passed through, and no longer repeat in the course of their growth. Take the case of the eye, for example; this in the young sea-squirt lies embedded in the brain, and is only dimly able to perceive light received through the transparent head; but the eye of all the backboned animals is really an outgrowth of the brain which has forced its way to the surface; here we see it in its primitive or original condition. The mouth in the young sea-squirt, again, opens on the top of the head instead of in the front, which is here modified to form a sucker. But the gills, by which this little creature breathes, expel the water by which they are bathed through a single hole at the side of the head, as in the frog tadpole; while in the possession of a brain, a spinal cord, and a soft backbone, both sea-squirt and tadpole agree.
Thus, then, the captor of one of these baby sea-squirts though he knew nothing of the peculiar after-history of the creature, would yet be sure that he had here a very young or 'larval' stage of one of the backboned animals. But he would be surprised indeed, as he watched the career of this little creature, to find it grow daily more sluggish, and at last fix itself by the sucker at the front of its head, and there remain as if in 'the sulks.' From this time onwards the change for the worse grows rapidly. This creature, as if indifferent to the great possibilities before it, or caring nothing for the good name of its race, speedily degenerates. As it will use none of the good gifts of Nature, one by one she takes them away—eye and brain are the first to go; then the tail begins to grow less and less (you can see the last remnants of a tail in fig. 3), and finally there is neither head nor tail, power of sight, nor power of motion; all that remains is an irregular-looking leathery lump, which scarcely seems to be alive (fig. 4). It feeds by drawing water through a hole at its upper end into a great throat pierced by gill-slits (shown in fig. 5, which represents a sea-squirt with the outside wall cut away); the water passes out through the slits into a big chamber. From this chamber the water escapes by another hole (marked 'discharge' in fig. 5) to the outer world again; meanwhile, the food, consisting of microscopic animals, has been caught by a moving rope of slime running along the back of the throat, and so into the stomach. But what a fall! Think of it—a career full of promise, the equal of that of vertebrate animals, ending in an ignominious surrender of its birthright, and a drop to the level of the humble oyster!
W. P. Pycraft, F.Z.S., A.L.S.
"'Some one is lost in the snow, and Lassie knows it.'"