COMPARATIVE ANATOMY AND PHYSIOLOGY. RADIATA. 247 



als, which it has to prepare for the nutrition of the body ; and those of the 

 muscles of the thorax, &c., maintain that constant interchange of air in the 

 lungs, which is necessary for the aeration of the blood : whilst those, by 

 which a limb is involuntarily retracted from any cause of pain or irritation, are 

 obviously adapted to the latter of these two ends. 



2. Comparative Anatomy and Physiology of the Nervous System in 



Invertebrated Animals. 



310. Although the structure and distribution of the Nervous System in the 

 different classes of Animals have been, until recently, but little appealed to 

 in the determination of its functions, they are capable of supplying evidence 

 regarding some of these, not less important in its character than that which 

 Comparative Anatomy affords to other departments of Physiology. Some of 

 the principal of these contributions will now be pointed out. 



311. In the lowest tribes of the RADIATED division of the animal kingdom, 

 no Nervous System has yet been discovered. These have, therefore, been 

 separated by some naturalists into a new primary group, to which the desig- 

 nation of Merita has been given, on account of the (supposed) "indistinct, 

 diffused, or molecular character of their nervous system." This idea of a 

 "diffused nervous system" seems to be regarded by many Physiologists as 

 well as Naturalists as the necessary alternative, resulting from the want of 

 any definite indications of its presence. It may be said, however, to be based 

 on very erroneous notions, as to the true offices of the nervous apparatus. 

 Its influence is not required to endow the tissues with contractility ; a pro- 

 perty possessed in a high degree by the structures of many Plants, to which 

 these beings present a much greater general resemblance, than they bear to 

 the higher Animals; and, even in the latter (as will be shown hereafter), this 

 property is independent of nervous agency, although generally called into 

 exercise by it. That a nervous system is not required by them for the per- 

 formance of the functions of Nutrition and Reproduction, otherwise than to 

 supply, by its locomotive actions, the conditions of those functions, would 

 also appear from its absence in Plants. It is on the sensible movements of 

 these beings, that our belief in their possession of a nervous system must be 

 founded, when we cannot render it cognizable by our senses. But we must 

 be careful not to draw hasty inferences from such phenomena. Sensible move- 

 ments are, as we have seen, performed by the Dionaea and Sensitive plant, in 

 respondence to external stimuli acting on distant organs ; and they are also 

 exhibited, in a very remarkable manner, by the reproductive particles of many 

 of the simpler Plants, as well as by numerous beings now generally referred 

 to the Vegetable kingdom. It is to be remarked, however, that such motions 

 are of a very simple description. In objects of the latter class, they are of 

 a rhythmical character, and do not seem to be in direct dependence on any 

 external influences. And even where they are performed solely in respond- 

 ence to external stimuli, there is usually such a uniformity in their character, 

 as indicates that the means by which the influence is propagated are of a very 

 mechanical nature. On the other hand, those movements of Polypes, which 

 are performed in respondence to external stimuli, are of a much more varied 

 character; and there are others, which seem to indicate a certain degree of 

 voluntary power, and therefore to display a consciousness of impressions made 

 upon the body. These phenomena, then, would lead us to suspect the ex- 

 istence of a Nervous System in the beings which exhibit them ; not, however, 

 in a "diffused" condition, but in the form of connected filaments. For, what 

 consentaneousness of action can be looked for in a being whose nervous 

 matter is incorporated in the state of isolated globules with its tissues? How 



