40 



GENERAL BIOLOGY 



consisting of a single large " ganglion " or nerve-knot, 

 from which ramify nerves in all directions. In 

 Ciona, a member of this group, if one of these siphons 

 is touched with a needle, both of them contract 

 almost simultaneously. If, however, the ganglion 



be snipped out with 

 a pair of scissors and 

 one of the siphons 

 be touched with a 

 needle, the one stim- 

 ulated will contract 

 at once, but only 

 after a considerable 

 interval does the 

 other siphon like- 

 wise contract. There 

 has been a conduc- 

 tion of the stimulus 

 in both cases. In 

 the first instance the 

 nervous system 

 afforded so perfect 

 a means of conduc- 

 tion that the contrac- 

 tion of both siphons 

 occurred almost together. In the second experi- 

 ment, however, the only path of conduction lay 

 through the intervening muscle fibers, specialized 

 along the line of contractility but not that of con- 

 duction. Hence the conduction was very imper- 

 fectly and slowly carried out. 



FIG. 16. dona intestinalis, a Tuni- 

 cate : a and b, the two siphons ; c, foot ; 

 d, location of the ganglion. (From 

 Loeb.) 



