ANIMAL PHYSIOLOGY 



67 



muscles and sense-organs with these central parts differs 

 accordingly. 



In the worms, Crustacea, and insects, which have much 

 the same body-plan, the central nervous system (fig. 26) con- 

 sists of a chain df ganglia (small nerve-centers) along the ventral 



portion of the body, this chain being 

 connected at the anterior end by a 

 cord on each side of the gullet, 

 with a large head ganglion which 

 stands in the position of the 

 vertebrate brain. In the starfishes 

 and sea-urchins, the central nerv- 

 ous system has the form of a ring 

 with radiating branches, but with 

 no head ganglia. In sea-anemones 

 and jellyfishes it is somewhat sim- 

 ilar, but is less distinctly set apart 

 from the other tissue-cells. In the 

 one-celled animals we recognize 

 Fig. 25. Brain of a cat, dor- no trace of a nervous system any 

 sal surface; I, olfactory more than we do of a muscular 



Sl^L;'ni:t2en:S; - ^-^^ ^y?^^-- ^ ^--^a the 



IV, medulla oblongata, whole Cell IS in a Weak way sensi- 



( After Reighard and Jen- tive to light, heat, jars, odors, acids, 



"'"^^■^ alkalis, and the various other things 



that affect the sense-organs of higher animals. The cell as 



a whole conducts the effects of these to all its parts and the 



response of the animal is slow and indefinite. 



In recent years a great deal of careful observation and 

 experimentation has been done on the behavior of the 

 simplest animals. The conclusions of the naturalists who 

 have done this work are not yet in sufficient harmony to 

 make possible any satisfactory generalizations, but it seems 

 certain that much of the behavior of the simpler animals is 

 determined and controlled by agencies outside of the body. 



