NERVOUS SYSTEM OF RADIATA. 479 



take a brief survey of its comparative structure in the principal groups 

 of Animals ; and to inquire what actions may be justly attributed to its 

 several divisions in each instance, commencing with those in which 

 the structure is the simplest, and the variety of actions the smallest : 

 and passing on gradually to those in which the structure is increased in 

 complexity by the addition of new and distinct parts, and in which the 

 actions present a corresponding variety. 



2. Comparative Structure and Actions qf ike Nervous System. 



848. From what has been already said ( 373-9) of the characters 

 of the two elementary forms of the Nervous tissue, it is evident that no 

 Nervous system can exist, in which both these forms should not be 

 present. We look, therefore, for ganglia, composed of the vesicular 

 nervous substance, and serving as the centres of nervous power ; and 

 for cords or trunks, composed of the tubular substance, and serving to 

 communicate between the ganglia and the parts with which they are to 

 be functionally connected. Now it is quite certain that, at present, no 

 such Nervous apparatus can be detected in many of the lowest Animals ; 

 and some Physiologists have had recourse to the supposition of their 

 possessing a "diffused" nervous system; that is, of their possessing 

 nervous particles, in a separate form, incorporated, as it were, with 

 their tissues. But we have seen that each tissue possesses its own pro- 

 perties, and can perform its own actions independently of the rest ; 

 that even the contractility of Muscular fibre is by no means- dependent 

 upon the Nervous system, though usually called into play through its 

 means ; and that the simplest office of a Nervous System is to pro- 

 duce a muscular movement in respondence to a certain impression ; 

 which action requires that it should have an internuncial or commu- 

 nicating power, only to be exercised (so far as we at present know) by 

 continuous fibres. The apparent absence of a Nervous system is 

 doubtless to be attributed, in many instances, to the general softness of 

 the tissues of the body, which prevents it from being clearly made out 

 among them. And it is to be remembered, that, on the principles 

 already stated, we should expect to find it bearing a much smaller 

 proportion to the entire structure, in the lowest Animals, whose 

 functions are chiefly Vegetative, than in the highest, in which the 

 vegetative functions seem destined merely for* the development of 

 the Nervous and Muscular systems, and for the sustenance of their 

 powers. 



849. Among the Radiated classes, the parts of whose bodies are 

 arranged in a circular manner around the mouth, and repeat each other 

 more or less precisely, the Nervous system presents a corresponding 

 form. In the Star-fish, for example, which is one of the highest of 

 these animals, it forms a ring, which surrounds the mouth ; this ring 

 consists of nervous cords, which form communications between the 

 several ganglia, one of which is placed at the base of each ray. The 

 number of these ganglia corresponds with that of the rays or arms ; 

 being five in the common Star-fish ; and from nine to fifteen, in the 

 species possessing those several numbers of members. The ganglia 



