2^6 THE STARFISH. PHYLUM ECHINODERM ATA 



Stable position, but on the receipt of a stimulus one of the five 

 centres becomes dominant, and impulses are sent round the 

 nerve-ring and along the radial tracts to cause all the tube-feet 

 to point and swing in the same plane. Conduction of the impulses 

 is decremental, so that an impulse caused by a weak stimulus 

 may die out before it reaches the other arms, and one of these 

 may in turn become dominant. The movements of the pedicellariae 

 and spines are controlled by local impulses in the superficial 

 nerve-net. This can also cause the tube-feet to respond to strong 

 stimuli. 



ECHINODERMATA 



The starfish is an example of the phylum Echinodermata, 

 a peculiar group which shows a mixture of primitive and highly 

 specialised characters. They are triploblastic and coelomate, 

 but have no blood system and the nervous system is a nerve net. 

 The water vascular system, a specialised part of the coelom, is 



Fig. 222. — Semi-diagrammatic views of a starfish (C), a sea urchin (B), a holo- 

 thurian {A), and a crinoid (D), in the natural position. — From Lang. 



a, Aboral side ; o, oral side. 



associated with the peculiar method of locomotion by tube-feet, 

 but is also excretory and perhaps respiratory in function. There 

 are various peculiar types of larva, which are bilaterally sym- 

 metrical, but the adults have a secondary radial symmetry, 

 replaced in one class by a tertiary bilateral symmetry. The original 

 plane of bilateral symmetry is marked by the madreporite. The 

 echinoderms, alone among invertebrates, have a mesodermal 

 skeleton ; this consists of calcareous ossicles. In this skeleton, 



