The mouth of the starfish is in the center of the ventral side. A ventral or /ambulacral groove/ extends through each arm. In these grooves the ambulacra, or tube-feet, are arranged in rows. The /ambulacra/ are hollow, cylindrical bodies, each of which has a sucker at one end, and is connected at the other end with a little globular body, the /ampulla/. The ventral groove forms a ridge in the body-cavity. It is called the /ambulacral zone/, and is formed by a double row of elongated plates, which meet and form a raised line along the middle of the inside of the arms, and appear like ribs; between them lie the ampullæ. These plates are called the /ambulacral ossicles/. Numerous other ossicles extend over the surface, buried in the integument of the body, making a calcareous network, or /exoskeleton/. The ossicles are connected by muscles so that the animal is not rigid, but is able to bend the body and even creep through comparatively small places. The ossicles are covered with /spines/ of two kinds. The very minute ones are the /pedicellariæ/, and have a jointed end which opens and shuts like a pincers. Their principal use seems to be to remove waste matter or other substances from the body, keeping it free and clean. The pedicellariæ are arranged, in some species, in circles around the [pg206] bases of the spines, and form groups over the dorsal surface. Examining the backs of different starfishes with a strong magnifying-glass, one will be surprised to see the varied grouping of the spines and pedicellariæ. At the extremity of each arm is a red eye-spot, which is sensitive to light, and a tube-foot, which has no sucker and which is believed to act both as an olfactory and as a feeling organ. The madreporic plate lies at the angle of two of the arms or rays. Water filters through the madreporic plate and passes by the stone-canal to the ring-canal around the mouth, and thence to the radial tubes, one of which extends through each arm and is connected by a branch with each tube-foot. At the base of each tube-foot there is a globular reservoir, or ampulla. A valve connects the tube-foot and ampulla. When the ampulla is contracted, water is forced into and distends the tube-foot; when the ampulla is distended, it withdraws the water from the tube-foot, thus acting like a suction-bulb. When water is sent into the tube-feet they are greatly extended and are then placed against an object. The water being then withdrawn through the ampullæ, a vacuum is formed, and the suckers are tightly attached. By the alternating movements of the ambulacra the animal is dragged slowly along and moves with a gliding motion. It does not deviate from a straight path, going over elevations or through depressions without turning aside; its body conforms to the irregularities of the road and never bridges over spaces. The stomach extends a little way into the arms, and a short intestine leads to the excretory opening in the center of the back.