If you will split a big one at this time of year you will be likely to find within an astonishing number of eggs. These are carried in brood pouches that seem to occupy pretty nearly all the space between the shells. In seeing them you wonder vaguely where there was room for the bearer of this amazing progeny. Just where they are these young unios grow to maturity of a certain sort, forming minute shells which have hooks, forming also peculiar organs of sense. The hooks and the sense organs are provided that they may not miss that free ride which is the privilege of every young unio if he is to reach the period of adolescence.
At the moment of being sent forth from the home shell the golchidium, for that is what the scientific men call the unio at this stage of the affair, begins to hunt, aided by his sense organs, for a thoroughfare. Here he takes the first conveyance, whether the slow coach of the sluggish horn-pout, the bream automobile, or the pickerel flying-machine. To the first fish that comes by he attaches himself, oftentimes to the gills, and there he rides and, like most travelers, continues to develop.
By and by, being “finished” by travel, he gets off his vehicle at some convenient station, drops into the mud, and is ready to lecture, or so I fancy it, before any of the unio women’s clubs on the world as he has seen it. Not until then does the unio, and then only if he is a margaritifera, begin to accumulate pearls.
By what mystery of sunlight and shallow water the unio has acquired the lucent green and gold of the epidermis of his outer shell I do not know, any more than I know what pigments paint or what naiad fingers hold the brush that paints the gold in the heart or the pinky green in the outer sepals of the water-lily. The two find their sustenance in the same mud.
But even if I could tell this I might well pause in wonder over the beauty of the inner shell of this pulseless creature of the ooze. Perhaps the golchidium, darting back and forth beneath the ripples of the surface during its days of travel, catches the radiant blue of the sky, the rosy flush of dawn, and the glory of the rainbow all shivered together in exultant light to make the nacre of the inner surface of its growing shell. For nowhere else in nature may we find such softness of coloring holding such gleams of azure and of fire. The opal beside it is garish and crude. Mother-of-pearl we call it, for out of the same source is born the gem which may be worth the price of a king’s ransom.