486 G. H. PARKER 
cle and toward the apical margin. In both these respects rachid- 
ial peristalsis has a most striking resemblance to the contrac- 
tion of the vertebrate heart. In this organ, as in Renilla, any 
part may orignate a contraction, but the rate at which such 
contraction may arise is different for different parts, being most 
oO 
= 
Fig. 6 A Renilla cut into transverse pieces to be tested for the rates at 
which the rachidial waves originate. The pieces may be designated, begin- 
ning at the bottom, as follows: peduncle, proximal rachis, middle rachis, sup- 
apical rachis, and apical rachis. (Compare table 3.) 
Fig. 7 A deeply incised Renilla in which the connecting bridge of tissue has 
been treated with magnesium sulphate (dotted circle). The extent of the 
rachidial waves is indicated by arrows. 
Fig. 8 A Renilla bisected except for the distal region of the peduncle. One 
half-peduncle is treated with magnesium sulphate (dotted circle). The two 
independent rachidial waves are indicated by arrows. 
rapid in the sinus, less so in the auricle, and least so in the ven- 
tricle. This has been demonstrated in the vertebrate heart by 
the same method as that used in Renilla, namely, by cutting the 
organ into separate pieces and determining the rate of each 
piece. If, in consequence of its rapidity of action, the sinus of 
the vertebrate heart may be regarded as the pace-maker for the 
