10 2 FIRST LESSONS IN ZOOLOGY 



position of the vertebrate brain. In the starfishes and sea- 

 urchins, the central nervous system has the form of a ring 

 with radiating branches, but with no head gangUa. In 

 sea-anemones and jellyfishes it is somewhat similar, but is 

 less distinctly set apart from the other tissue-cells. In 

 the one-celled animals we recognize no trace of a 



P'lG. 67. 



Fig. 67. — Nervous system of the house-fly, the central nerve-cord and 

 ganglia lie in the ventral (under) part of the body. (After Brandt.) 



Fii:. 68. — Nervous system of a midge {^Chiroiiomiis)-^ note the separation of 

 all the ganglia. (After Brandt.) 



nervous system any more than we do of a muscular or 

 bony system. 



In comparing the nervous systems of various animals 

 as we have their muscular, bony, and circulatory systems 

 wc find the same variety and progressive degree of 

 development holding true. In Amceba the whole cell is 

 in a weak way sensitive 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. 



