NOTES ON AN AMPHIOXÜS OBTAINED IN AMAKUSA, KYUSIIYU. 129 



VI. The Position of the Tail-Fin, the Number 

 and Distribution of Muscle-Segments, and the Formula for 

 the Present Species. 

 The number of segments was always counted only on the left side. 

 The segment in which the anterior end of the dorsal or ventral portion 

 of the tail-fin commences was counted from behind forward. 



In determining the position of the atriopore I have met with no 

 little difficulty, but as the posterior end of the metapleural folds almost 

 always corresponds with the hind margin of the pore, I have taken that 

 end as the landmark lor distinguishing the anterior and middle portions 

 of the three regions into which the body may be divided. For the 

 anterior limit of the posterior region I have taken that segment as the 

 first which lies just behind the projecting lip of the anus. The following 

 table shows the results of these observations. 



TABLE IV. 





No. of 



No. of 



No. of 



No. of 



No. of 





segments in 





segments in 



segments be- 



segments be- 



Body 



segments in 



hind the com- 



hind the com- 





the 





the 



mencement 



mencement 





anterior 



the 



posterior 



of the 



of the 



length. 



dorsal end of 



ventral end of 





region. 



middle region. 



region. 



the tail fin. 



the tail fin. 



Specimens possessing G6 segments in all. 



49 ... 



... 38 ... 



... 17 ... 



... 11 ... 



... 11 ... 



... 17 



37 ... 



... 37 ... 



... 18 ... 



... 11 ... 



... 13 ... 



... 18 



Specimens possessing 65 segments in all. 



481... 



... 38 ... 



... 16 ... 



... 11 ... 



... 11 ... 



... 17 



441... 



... 36 ... 



... 18 ... 



... 11 ... 







421... 



... 37 ... 



... 17 ... 



... 11 ... 







4H... 



... 37 ... 



... 17 ... 



... 11 ... 



13 ... 



18 



41i... 



... 37 ... 



... 17 ... 



... 11 ... 



... 12 ... 



... 18 



40 ... 



... 37 ... 



... 17 ... 



... 11 ... 



... 11 ... 



... 17 



39 ... 



... 37 ... 



... 17 ... 



... 11 ... 



... 12 ... 



... 17 



361... 



... 37 ... 



... 17 ... 



... 11 ... 







36 ... 



... 38 ... 



... 18 ... 



... 9 ... 



" . . . 13 . 



17 



35 ... 



... 37 ... 



... 17 ... 



11 ... 



... 12 ... 



... 17 



33 ... 



... 36 ... 



... 17 ... 



... 12 ... 



... 12 ... 



... 18 



291... 



... 37 ... 



... 17 ... 



... 11 ... 



... 11 ... 



... 17 



