NERVOUS SYSTEM OF IN VERTERRATA. 48.3 



§ 2. Nervous St/stem of Invertebrate Animals. 



No trace of a nervous system can be discerned 

 in the animals occupying the very lowest place 

 among Zoophytes, such as Sponges and Polypi. 

 In textures of such extreme softness, indeed, as 

 that which composes the fleshy portion of these 

 animals, it would be difficult to detect nervous 

 fibres, if any existed ; for they would probably 

 partake of the general transparency and want of 

 cohesive tenacity which characterizes every part, 

 and thereby elude research. The indications of 

 sensibility which they afford are very obscure ; 

 for the contractions they exhibit, when stimulated, 

 appear to be rather the effect of a vital property of 

 irritability than the result of any sensorial faculty. 

 Analogy, however, would lead us to the belief that 

 many of their actions are really prompted by 

 sensations and volitions, though in a degree very 

 inferior to those of animals higher in the scale of 

 being ; but whatever may be their extent, we can- 

 not perceive that the sensorial operations in these 

 animals take place through the intervention of any 

 common centre of action. It is at the same time 

 remarkable that their movements are not effected 

 by means of muscular fibres, as they are in all 

 other animals ; the granular flesh, of which their 

 whole body is composed, appearing to have a 

 generally diffused irritability ; so that each isolated 

 granule seems to be capable of performing, inde- 

 pendently of the other granules, the functions both 

 of nerve and muscle. Such a mode of existence 



