THE PHYLA HEMICHORDATA AND ECHINODERMATA 



367 



epidermis is ciliated and the ciliary currents continually sweep the fine 

 debris that settles on the starfish out to the sides where it falls off. The 

 larvae of molluscs, barnacles, bryozoans and others that might attach 

 to the naked spines are discouraged by the pedicellariae (Figs. 19.6 E 

 and 19.8 A), each a microscopic pincers. These snap vigorously when 

 stimulated and any pedicellaria that catches anything remains shut for 

 several days. These are scattered over the body surface and clustered in 

 rosettes at the bases of spines. Occasionally the tissue around the base 

 of a spine contracts, lifting the rosette so that the pedicellariae reach to 

 the tip of the spine, snapping all the way up and down to clean its 

 surface. In this way the starfish does not become a travefing home for 

 attached organisms. 



Asterias creeps slowly on a multitude of tube feet, delicate pro- 

 jections ending in suckers. These project from deep ambulacral grooves 

 radiating from the disc along the lower surface of each ray. The tube 

 feet are arranged in two longitudinal rows, each of which is staggered 

 so as to look like a double row. Their epidermis is not ciliated. 



Each tube foot operates as an independent hydraulic mechanism 

 (Fig. 19.8 B). Its cavity, which is a part of the coelom, extends inward 

 through the body wall and expands inside the body as a bulb or am- 

 pulla. W^hen the muscular coat of the ampulla contracts, the fluid in 

 the cavity is forced into the foot. Since the wall of the foot contains 

 connective tissue rings that prevent expansion of the tube diameter, the 

 foot elongates as it fills. The end ot the foot forms a suction cup, and 



'^Ampulla. 



"Jaw 



Closing muscle 

 Opening iiiuscle 



Tube, 

 foot 



Te.nta.cle 



RadiaJ canal 



Lza.-tei'a.l Ccoial 



itudinal miLScle 



Circular connective 

 tissue, fibers 



Muscle fibers 

 (increase Su-Ction) 



Connective, tissue 

 spray 



Nerve tissue 



•Eye spots 



Figure 19.8. A, One of the several varieties of pedicellariae. B, The tube foot and 

 associated apparatus. C, Section through a terminal tentacle (suckerless tube foot) showing 

 the eyespots at its base. (All figures diagrammatic, modified from Hyman.) 



