a given direction as long as the cilia upon its surface continue their mysterious activity."
In the oysters and scallops, the mantle is open, exposing the gills; while the mussels and giant Tridacna have the mantle closed but pierced with two apertures for the reception and expulsion of water. The remaining lamellibranchs, which include the common hard clam, razor-shell, pholades, and teredo, have long inhalent and exhalent tubes, called siphons, which enable the creature to burrow in the sand and still obtain water. The two siphons may be separate or united, and are retractile. The presence of such tubes can be determined by examination of a dead shell; as the muscles which retract the siphons produce an indentation or "sinus" in the "pallial line."
Perhaps the most conspicuous and beautiful gills are those possessed by the naked sea-snails or "sea-slugs." They are situated on the back and sides of the body, and are frequently retractile. In the Doris they have a flower-like or star-shaped arrangement. In the Eolis they are papilliform and tufted, along the sides of the animal, or they may be tree-like as in the Triton, or feathery in other forms. The Aphysia, or sea-hare, has the gills placed on the back and protected by a fold of the mantle. The Phyllidia have them as a fringe along each side of the body, and covered by a projecting fold of the skin; while the Limpet and Chiton have leafy gills forming quite an entire circle about the body, and also covered by the mantle.
The sea-snails having shells carry their gills in cavities in the side of the neck, and may or may not have siphons. The possession of the latter is shown by a notch in the aperture of the shell.
All the mollusks mentioned thus far are sluggish animals, with little need of rapid respiratory changes of the blood. But the highest of mollusks, the cephalopods, or devil-fishes, are active and muscular creatures—the most powerful of invertebrates. They have accessory hearts to accelerate the flow of blood through the gills, which are large and plumose, and contained in a cavity. Water is freely sup-
- T. Rymer Jones, "The Animal Creation," page 194.