. The physiology of the Invertebrata. above,only the motion is very rapid. It appears, however, as ifthe feet, besides being used as walking poles in the mannerjust described, are also used to sweep backwards along thefloor of the tank, and so to assist in propelling the animalforwards after the manner of cilia. Therefore, while walkingin water, Astropecten is kept stilt-high above the surface onwhich it is walking, by some of its feet, while others of itsfeet are engaged in these sweeping movements. Astropecten has a rapid rate of movement, being betweenone and two feet per minute. When place
. The physiology of the Invertebrata. above,only the motion is very rapid. It appears, however, as ifthe feet, besides being used as walking poles in the mannerjust described, are also used to sweep backwards along thefloor of the tank, and so to assist in propelling the animalforwards after the manner of cilia. Therefore, while walkingin water, Astropecten is kept stilt-high above the surface onwhich it is walking, by some of its feet, while others of itsfeet are engaged in these sweeping movements. Astropecten has a rapid rate of movement, being betweenone and two feet per minute. When placed upon its back, ithas the power of righting itself very rapidly. * The feet usually remain extended for a quarter to half a minute, but-very suddenly collapse; PHYSIOLOGY OF THE INVERTEBRATA. 383 {d) The OpMuridea.—In the brittle-stars the ambulacralfeet are only rudimentary, although exceedingly active; theyare devoid of suckers ; and their mode of protrusion and re-traction is exactly like that of Astropecten, but more rapid in. >< I «.< w Z< a o S•^ o action. These animals are much the most actively locomotiveof all the starfishes; and the reason is, that having discardedthe method of crawling ambulacral system, which iscommon to nearly all the other Echinoderms, they haveadopted instead a completely new, and a much more effectual 384 PHYSIOLOGY OF THE INVERTEBRATA. method. The muscular system of the rays is very peideveloped, enabling these long and snake-like appendaperform with energy and quickness a great variety of slike writhings. As the movement of all the arms is conated, the animal is able by these writhings to shufflealong flat horizontal surfaces at a considerable speed,when it desires to move still more rapidly, it adopts aiplan. If the animal is advancing in the direction carrow (Fig. 75), one of its rays, i, is pointed straight!direction; the two adjacent rays, 2 and 3, are throw:wards as far as possible, and then, by a strong contrdown
Size: 1323px × 1888px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectinverte, bookyear1892
