GENERAL ZOOLOGY 



exchange and in excretion. Clustered at the bases of the spines and among 

 the papulae are minute, pincer-like structures, the pedicel lariae, which func- 

 tion in keeping the surface free of foreign matter. In such a sluggish, slow- 

 moving animal as the starfish, this is a very important function indeed. In 

 addition to finely divided particles of silt and detritus, the ocean is full of 

 minute larvae of such forms as sponges, coelenterates, encrusting ectoprocts, 

 and barnacles, seeking surfaces on which to settle and produce their character- 

 istic growths. Starfishes are never encumbered by such growths, probably be- 

 cause of the activities of pedicellariae in removing them and keeping the 

 papulae free of obstructions. 



At one side of the disk, between the bases of two of the arms, lies a porous 

 plate, the madreporite or sieve plate; through its minute openings the internal 

 water-vascular system communicates with the exterior. For convenience in 

 reference, the two arms between which the madreporite lies are termed the 

 bivium; the other three are the trivium. On the oral surface the mouth occupies 

 the center of the disk, surrounded by an oral membrane or peristome. Radiat- 

 ing from the mouth along the arms are five ambulacral grooves, from which 

 project numerous locomotor organs called tube feet. At the outer end of each 

 ambulacral groove lies a small, reddish eye spot below a short sensory tentacle. 



The starfish creeps slowly along the bottom by coordinated stepping move- 

 ments of its tube feet. Although it seems inflexible, and its arms may be 

 broken off by rough handling, the animal can bend and twist in a great 

 variety of ways. For example, when a starfish is turned upside-down, it rights 

 itself by twisting the arms until some of the tube feet become attached to the 

 bottom. Using these attached points for traction and for reference, the 

 animal slowly turns itself back to the normal position. Changes in shape and 

 attitude are brought about by the action of muscles which interconnect the 

 calcareous plates of the body wall. Normal locomotion, however, is effected 

 primarily by the tube feet, which in Astenas terminate in suckers and can be 

 firmly attached to a hard substrate. Under certain conditions these suckers 

 seem to be necessary, as when the animal clings to a wharf pile or stone or 

 walks up the glass wall of an aquarium. But the animal can walk perfectly 

 well without attaching its suckers, over a loose, sandy bottom or upon a 

 greased surface, and the tube feet of many species of starfishes lack suckers 

 altogether. The mechanism by which the tube feet operate will be explained 

 in connection with the ambulacral system of which they are a part. 



There is nothing like a head or an anterior end in the starfish; the animal 

 can travel in any direction, and no part appears to assume the lead more 

 frequently than others. Once the animal has started in a particular direction, 

 however, the tube feet of all the arms step in the direction taken by those of 

 the leading arm. The starfish thus progresses steadily until it encounters some- 

 thing to change the course of locomotion. A comparable coordination is 

 shown in the righting reaction, when all the arms and their tube feet move in 

 an integrated fashion as soon as a firm hold has been secured by some of the 

 tube feet of one of the arms. 



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