PHYLUM ECHINODERMATA 195 



tions with their thin walls swinging about freely in the 

 water. The branchiae are minute, finger-like organs which 

 are scattered plentifully over the whole upper (aboral) 

 surface of the starfish. They are hollow and connect 

 directly with the coelom. Cilia keep currents passing in and 

 out of the branchiae, and the coelomic fluid is thus aerated 

 through the thin membranes exposed to the sea water. 

 Asterias has no special means of excretion, except little 

 amoeboid cells* which wander about in the tissues engulfing 

 waste products. These finally migrate outside the body 

 with their loads. 



Self-protection. Asterias is protected from most prer 

 daceous animals by the hard, spiny skeleton. When left 

 stranded at low tide, however, its watery body readily 

 dries up. A starfish crawls boldly over the rocks at high 

 tide, but creeps into crevices and hides when the ocean 

 ebbs. There is a simple eye at the tip of each ray and the 

 whole surface of the body is more or less sensitive to light ; 

 so Asterias can tell when it is in light or shadow, and is 

 able to crawl toward dark spaces. 



A starfish moves very slowly and the body gathers many 

 particles of dirt which fall from above. Such accumula- 

 tions are continually swept to the edge of the body and off 

 by the cilia which cover the soft parts between the spines. 

 There are many very small animals in the ocean which 

 might gnaw the branchiaB and the flesh of the starfish if it 

 were not for the presence of little seizing organs, the 

 pedicellarise, which are to be found around the bases of the 

 spines. Each pedicellaria consists of a short stalk sur- 

 mounted by a pair of calcareous pinchers. If a small 

 animal starts to walk over the starfish, it is seized and held 

 immobile until it dies (Fig. 80, C). The pedicellariae are 

 rather independent in reacting and will often pull against 

 each other. They grasp anything that moves and hold on 

 until they are jerked loose from the body, or until the thing 

 they hold ceases to move. Through their activities the 



