GENERAL ZOOLOGY 



external sea water through the papulae, which thus function also as excretory 

 organs. In general, it is clear that the circulating coelomic fluid performs 

 the major functions which in most other animals are subserved by the cir- 

 culating blood. This is particularly interesting, in view of the fact that, 

 aside from small amounts of proteins and nutrients dissolved in it, the 

 coelomic fluid is almost identical with sea water in its composition. In fact, 

 certain organs of the starfish, notably isolated pyloric caeca, will survive 

 for several days in cool, aerated sea water. 



The nervous system of the starfish is basically organized on the radial plan 

 typical of other parts of the body. Its chief components are a circumoral nerve 

 ring, surrounding the peristomial membrane; a series of five radial nerve cords, 

 one in each arm, extending from the nerve ring; and a generally distributed 

 subepidermal nerve plexus with connections into the radial cords. In addition 

 to the conspicuous eye spots and sensory tentacles at the tip of each arm, 

 receptors are scattered throughout the epidermal layer. The nerve ring and 

 radial cords, superficially located, consist of thickened and specialized areas 

 of the epidermal layer (Fig. 16.4). They contain many neurons, arranged 

 in sensory, association, and motor tracts. Aside from the circular and radial 

 nerve cords, there are no true nerves in the starfish; nerve cell bodies are 

 not restricted to the cords, and nerve fibers pass out from the cords more or 

 less individually. The receptors that occur in the epidermis send afTerent 

 branches into the plexus layer, from which motor fibers run directly to 

 muscles of the papulae, pedicellariae, and spines. Localized reflex activities 

 of these structures are thus possible without the intervention of the "central" 

 nervous system. More generalized responses involve afferent fiber tracts from 

 the plexus into the radial cords, and complex eflferent or motor pathways in- 

 volving series of neurons that course from the cords to specific muscles 

 of the body wall and tube feet. At the central end of each radial cord, the 

 nerve ring contains a "motor center," a large group of nerve cell bodies 

 which appear to be responsible for coordinating the activities of the tube 

 feet in all the arms. In locomotion, the assumption by one arm of the "lead" 

 position seems to involve a temporary dominance by the motor center of the 

 leading arm over all the other motor centers. This condition is transitory, 

 however, and the "lead" passes to other arms and their centers in turn. 



The wall of the digestive tract contains a conspicuous nerve plexus layer, 

 which undoubtedly has connections with visceral receptors and also with 

 the muscle layers of the gut wall. The functions of this part of the nervous 

 .system have never been analyzed. Although the nervous system of the star- 

 fish presents many peculiarities, both structural and functional, the operation 

 of this sytem is apparently fundamentally comparable with that of the nervous 

 systems of other metazoans. 



A unique feature of the anatomy of all echinoderms is the water-vascular 

 system, or ambulacral system (Fig. 16.5). Through the projecting tube feet, 

 the ambulacral system of the starfish functions chiefly in locomotion and in 

 adherence to the substrate, although it may contribute significantly to the 



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