. Animal studies. ECHINODERMS 157 tube-feet, which, as we have seen, are slender tubular or- gans, many in number, filling the grooves on the ventral surface of each arm. This entire system of tubes and reservoirs is full of water, taken in, it is said, through the perforated plate, and, when the starfish wishes to advance, many of the little reservoirs con- tract, forcing water into the cav- ity of the feet, with which they are in communication, thus ex- tending the extremity of the tubes a considerable distance. The terminal sucker of each foot, act- ing upon the same principle as those on t
. Animal studies. ECHINODERMS 157 tube-feet, which, as we have seen, are slender tubular or- gans, many in number, filling the grooves on the ventral surface of each arm. This entire system of tubes and reservoirs is full of water, taken in, it is said, through the perforated plate, and, when the starfish wishes to advance, many of the little reservoirs con- tract, forcing water into the cav- ity of the feet, with which they are in communication, thus ex- tending the extremity of the tubes a considerable distance. The terminal sucker of each foot, act- ing upon the same principle as those on the cuttlefish, attaches firmly to some foreign object, whereupon the muscles of the foot contract, drawing the body toward the point of attachment. This latter movement, is similar to that of a boatman pulling him- self to land by means of a rope fastened to the shore. When the shortening of the tube-feet has ceased, the sucking disks release their attachment, project them- selves again, and this process is repeated over and over. At all times some of the feet are con- tracting, and a steady advance of the body is the result. This method of locomotion also obtains in the sea-urchins and cucumbers, but in the serpent-stars the tube-feet have become modified into feel- ers, and the animal moves, often rapidly, by means of twist- ing movements of the arms. The feet have this character also in the crinoids, where the animal is generally without. Fig. 96.—Sea-iuy or Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Jordan, David Starr, 1851-1931; Kellogg, Vernon L. [from old catalog].
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Keywords: ., bookauthorjordanda, bookcentury1900, bookdecade1900, bookyear1903
