MOLLUSKS. These massive animals have to solve a much harder problem in respiration than have the Scolecida. In the lamellibranchiates the foot is surrounded by a double row of tenta cles. These remain as distinct straight filaments in a few genera, such as Nucula, Leda, Yoldia, and Solenomya ; but each filament is reflected, making a knee-bend, so that each series of fila ments or 'gills' is double, as in Anomia, Arca, and the mussels (Mytilid:e). The reflected part of each filament may be united with the basal part, and the free end of the reflected filament may grow fast to the body or to the mantle. In the other lamellibranchs the adjacent fila ments are united by cross-bars, forming a sort of network. Each filament and connecting bar is hollow; blood courses through it, and receives oxygen from the water that rushes by it on all sides. The mechanism for bringing the water to the gills is simple—water rushes into the mantle chamber, bathing the gills and penetrating be tween the filaments as it goes in and out. In
the shelled gastropods there is a pair of gills on the right and left of the neck in a few sym metrical species; but in most of the spirally coiled species there is only one gill, and that is on the left side. The gill consists of a finger like process containing a vessel carrying blood to the tip of the gill, and one carrying it from the gill. In passing from the first to the second vessel, the blood is spread out over numerous thin-walled plates, where it comes in contact with the water. In the naked gastropods respira tion occurs chiefly on the whole surface of the body, but sometimes by special finger-like out growths. In cephalopods there are either two or four gills, which lie in the mantle-cavity, pro jecting forward, and are fastened on both sides. The general arrangement is the same as in the gastropod gill, hut the capillary absorbing sur faces are much increased in area.