374 GENERAL STRUCTURE AND FUNCTIONS 



ately beneath it the stomach or digestive cavity, which sends pro- 

 longations into every one of the projecting limbs. There is also 

 contained in each limb a portion of the glandular and muscular 

 systems, and the whole is covered by a sensitive integument. The 

 nervous system consists of five similar ganglia, situated in the 

 central portion, at the base of the arms. These ganglia are con- 

 nected with each other by commissures, so as to form a nervous 

 collar or chain, surrounding the orifice of the digestive cavity. 

 Each ganglion also sends off nerves, the filaments of which are 

 distributed to the organs contained in the corresponding limb. 



"We have already stated that the proper function of the nervous 

 system is to enable a stimulus, acting upon one organ, to produce 

 motion or excitement in another. This is accomplished, in the 

 starfish, in the following manner : 



When any stimulus or irritation is applied to the integument of 

 one of the arms, it is transmitted by the nerves of the integument 

 to the ganglion situated near the mouth. Arrived here, it is 

 received by the gray matter of the ganglion, and immediately con- 

 verted into an impulse which is sent out by other filaments to the 

 muscles of the corresponding limb; and a muscular contraction and 

 movement consequently take place. The muscles therefore contract 

 in consequence of an irritation which has been applied to the skin. 

 This is called the "reflex action" of the nervous system ; because the 

 stimulus is first sent inward by the nerves of the integument, and 

 then returned or reflected back from the ganglion upon the muscles. 

 It must be recollected that this action does not necessarily indicate 

 any sensation or volition, nor even any consciousness on the part of 

 the animal. The function of the gray matter is simply to receive 

 the impulse conveyed to it, and to reflect or send back another ; 

 and this may be accomplished altogether involuntarily, and without 

 the existence of any conscious perception. 



Where the irritation applied to the integument is of an ordinary 

 character and not very intense, it is simply reflected, as above 

 described, from the corresponding ganglion back to the same limb. 

 But if it be of a peculiar character, or of greater intensity than usual, 

 it may be also transmitted by the commissures to the neighboring 

 ganglia ; and so two, three, four, or even all five of the limbs may 

 be set in motion by a stimulus applied to the integument of one of 

 them. Now, as all the limbs of the animal have the same structure 

 and contain the same organs, their action will also be the same; 

 and the effects of this communication of the stimulus from one to 



