Page:Popular Science Monthly Volume 8.djvu/196

184 base, which springs from a creeping gelatinous thread. The mantle is transparent as crystal, and through it you may see, as if suspended in the cavity of the body, what seems the frilled edge of a ribbon of snow-white lace. This is Clavelina's lung. A little sac, seen through the transparent mantle and body walls, contracting and expanding with a slow and measured beat, is Clavelina's heart.

Another cloaked mollusk is Cynthia. It adheres to rocks or pebbles under a few fathoms of ocean, and has something of the form and color of a blood-peach. It is known to watermen under the name of "sea-peach." Its mantle is tough and leathery.

Another and a more interesting member of the cloaked family is the Salpa. In the structure of the heart it marks' an advance on Clavelina. Instead of a single pulsating sac, we find an auricle and a ventricle, veins and arteries. But, Nature having advanced from a single to a double heart, it would seem that she did not yet know how to vise the improvement. In the Salpa we find the heart incessantly changing its auricle into a ventricle, its ventricle into an auricle, veins into arteries, arteries into veins.

The Salpæ swim freely in the open sea and occur singly, or united in long chains or rings. They are phosphorescent, and a chain of united Salpæ appears like a writhing, fiery serpent gliding over the waves. The Pyrosomes, which are free Salpæ, congregate in vast shoals, and in their phosphorescence glare like pillars of fire, green, unearthly, elfish.

Let the edges of the mantle unite along part of their surface, but remain open at the ends. The animal now will not be completely tunicated. It will be inclosed in a kind of funnel. If, now, such a mantle be drawn out into a siphon to conduct a current of water to the gills, it would be of use to the animal in aiding respiration. The edges of the mantle having united in this way, a siphon-bearing mollusk, like the cockle or solen, would be simply a question of time. Natural Selection would bring it about.

Let the edges of the mantle not unite at all, we shall have a mollusk something like the oyster.

Remove the shell, and an oyster lies before you in irregular, ragged outline. An opening at the sharper end, which lies near the beak of the shell, is the mouth. Around the mouth are four leaf-like bodies, which hang in pairs. The heart is an advance on that of Salpa, not in structure but in behavior. It has settled down into regular work, the auricle always an auricle, and the ventricle always a ventricle. The liver is a decided advance on that of Eolis, although not yet a well-defined gland. The mantle is a fringed, veil-like membrane,