534 Dynamic Theory. 



CHAPTER LYI. 



NERVOUS SYSTEM. 



Rhizopods, Amoebae, &c. , have no nervous system, the whole of their 

 substance being contractile, and all possessing nervous properties dif- 

 fused equally through it, and not differentiated from it. The Hydra, or 

 fresh water polype, has the power of contracting its parts by very sim- 

 ple muscles lying between the exoderm and entoderm, or inner and outer 

 skin layers, and it possesses sensibility or irritability, by means of which 

 stimulations may cause the movement of all the parts at once, and its 

 movements are correlated with each other, but yet there is no nervous 

 system in the animal. The Sea Anemone, or Actinia, another ccelenter- 

 ate animal, possesses a row of sensitive tentacles around the mouth, and 

 its substance is very contractile, and also quite sensitive to touch and 

 light irritations. Yet it has no nerves. During bright sunlight its ten- 

 tacles are spread out like the petals of a flower, but if a cloud passes 

 over they are folded up and retracted, and the mouth closed. Even the 

 shadow of a hand will produce this effect. Its excretion of the waste 

 products of its food materials is usually by way of the mouth by which 

 they entered ; but if a piece of bone or shell happens to be a little un- 

 manageable, it is pushed out through the side of the body wherever it 

 is most handy, and the rent thus formed is speedily repaired. That an- 

 imals so little differentiated as that, are still so sensitive to light and 

 touch, and that the stimulations are able to affect all parts of the body, 

 even those not directly stimulated, prove that the cells are themselves 

 sensitive organs, and able like nerves to convey stimulation to other 

 parts. Among the lowest orders in which a well defined nervous sys- 



FIG. 255. Diagram of nervous system of 

 Starfish. * 



N. Nerve ring which connects together the 

 five ambulacral centers or foot ganglions. 



tern is found, are the Echinoderms in- 

 cluding the Encrinites, Starfishes, 

 Medusae, Sea urchins, Trepangs, &c. 

 These animals, when adult, are con- 

 structed on the radiate or star plan, 

 both muscles in the form of tentacles 

 or arms, and the accompanying nerves 

 FIG. 255. radiating from a common center. In 



the lower orders of worms ( Scolecida ) we find the nervous system con- 

 sisting of one or two ganglia, the general protoplasm of the body still 

 possessing, in some degree, an undifferentiated conducting capability. 



